I'll go back to Johnny's original question and try to look at it in the wider context. Probably Riddles most important task - aside from actually providing the wherewithal to keep traffic moving - was an organisational one. He and the senior members of his team and lived through the organisational mess and infighting which followed the creation of the LMS at the Grouping and there can be little doubt that he worked hard and seemingly quite skillfully to avoid that sort of debacle. Stage 1 was the 'competition' of the 1948 Loco exchanges and trials - aimed primarily at producing some technical information but also undoubtedly seen as a sort of sporting contest as well and intended to shake folk up a bit.
That might seem, and no doubt was, as much competitive as 'welding together' but it started the exchange of ideas and experience which went into the standard design process - which also included coaching stock and wagons of course. Similarly to the loco design process certain works were given leadership on various parts of those schemes but working in teams with representatives from other works. Exactly the same happened with engines where features from the various companies were chosen to be developed into standard components for the new range of locos with lead drawing offices for various tasks but working in concert with other drawing offices. It was a clever practical example of what in modern terminology is called 'team building' and it helped to breakdown barriers and bring people together.
Why steam? Well here all the same reasons come out as identified by Cox - diesels were very much an unknown quantity of which there was very limited operational and maintenance experience of the emerging mainline types in 1948 plus there was the oil question and the shortage of US$ to buy it while the country was sitting ion a massive long term stockpile of underground coal. The logic identified steam asa stop-gap until electrification - but shortage of investment capital clearly meant that was a long way off (and much of it still is).
The other advantage of steam was that it was simple to design and build and railway staff knew how to do it (and, what a bonus, they could grow to know each other better in that process). Steam very simply made practical and economic sense in the conditions of the time. Even if there had been a trial scheme of dieselisation on a greater scale than the few emerging prototypes there was still a need to replace an ageing and inefficient fleet of steam engines fairly quickly. And the diesels would effectively be ordered off the drawing board relying on gaining some benefit from past export designs (and look how some of that turned out just over a decade later). Look too that when the Modernisation Plan high horsepower (i.e. 2,000 hp) locos emerged the d-e versions used a bogie design which had first entered traffic use in 1951 - so wasn't available before then except on the drawing board but of course it turned out to be pretty successful.
So those are I think the practical reasons for sticking with steam for mainline use (diesels for shunting were a slightly different issue).
Now what about the engines which actually emerged - well 4 classes (73XXX, 76XXX, 78XXX, and 84XXX) were basically titivated LMS designs with standard components where appropriate, some changes on the two Class 2 to improve Route Availability from a clearance viewpoint, and the standard style' adopted. The 80XXX tank engines were a far more serious re-working of the LMS design in order to widen route availability for clearance reasons. All of these types had continued in construction post 1948 in several cases at other than LMS workshops and were already in use on other Regions - logically further change might be considered unnecessary except in relation to route availability or perhaps in benefitting from improved standard components instead of the originals. The 75XXX Class 4 4-6-0 also had its origins in the same LMS tank engine design but was intended from the outset for lines where weight restrictions precluded use of the larger 73XXX type and where the higher power of the Class 5 wasn't needed - an order for a further10 was cancelled.
The 82XXX tanks were basically a design driven by axleload requirements and developed around the Swindon No.4 boiler suitably updated to BR Standard format and there was clearly, and immediately, work for them replacing - in many cases - ageing Pre-Group engines, those ordered in the 1954 building programme were cancelled. The 77XXX 2-6-0 was no more than a tender engine version of the 82XXX tank so involved little extra design cost and was again a design intended for weight restricted routes.
Most of the above probably had reasonable justification for their construction, even (almost?) the numerically small 77XXX class although its utility was, like many of the smaller classes, overtaken by a mixture of route improvements, closures, and - later - dieselisation.
That leaves several others of course and here is where some question marks exist in my mind. The 'Britannias' were a good design - once the various teething problems had been resolved - but seem to have been as much as anything a justification exercise to prove a powerful 2cylinder design could be built and maintained for lower cost than an equivalent 3 or 4 cylinder engine and be useful on work beyond the capabilities of a Class 5. In other words they were there to prove Riddles, and his immediate team's ideas on construction and maintenance. Effectively they added penny packet numbers in a number of places which were short of power but only really opened a new page in terms of train running on the GE mainline - elsewhere they were engine sin search of work and they often only got it as older classes fell by the wayside. The needs they served - except on the GE - could have been met by building more engines of existing designs BUT they would have cost more so the design had some sense to it but it is - in my mind - questionable that maybe too many were built.
The 'Clans' are an enigma - they seem to have been very much built to an outmoded concept and as a result were neither one thing or another although they did what they were supposed to do although oddly their reduced axleload (compared with the 'Britannia' was never exploited and it is hardly surprising that the final 15 were cancelled during Riddles period in office.
The real oddball was of course 'Duke of Gloucester' - if the Britannia could be considered to be a design to prove a point (which it generally did) then this was one was probably the nearest Riddles came to a true vanity project. There has long been some controversy about just how unrepairable 46202 was after the Harrow collision when it was far from being the most heavily damaged engine and it reportedly lingered for a long time at Crewe before finally being broken up. But that breaking up left room for a new Class 8 pacific and away went Riddles design team with the chance he and they no doubt had looked for. Alas it has only really been got right in its final preserved life as it never did particularly well in BR days; to me, and looked at in the wider sense, its the special financial authority to build it and its actual construction does seem rather questionable.
Finally we come to the 9F a brilliant design and a superb engine. It came late because there was no need for it any earlier but because of that late arrival and the drastic changes in traffic levels which grew out of the 1955 ASLE&F strike plus. later, the onset of mass dieselisation, it suffered the indignity of being an engine out of its time. A great pity but reasonably in my view something which could not necessarily have been foreseen when most of the class were ordered.
So all in all the Standards were rather mixed bunch - some were desperately needed when they were designed and built, some were already out of place by the time they arrived, and some simply suffered the consequences of totally unforeseen events. But in traffic almost all of them offered savings compared with many comparable designs and taht should not be overlooked or forgotten when judging their effectiveness.