Re loading gauges, read this: https://www.rssb.co.uk/Library/groups-and-committees/2013-guide-vehicle-structure-sic-guide-to-british-gauging-t926.pdf
As it points out, various changes to accommodate wider and taller containers, plus vat]riots other factors, mean that steam locomotives are now classed as “out of gauge” for general us, requiring clearance for specific routes, and the removal of “go anywhere” permissions.
Also, the key issue for expanding the U.K. loading gauge is not so much the time and cost for widening the envelope, but the impact this would all have on existing stock and new stock: it would all require power-operated retractable steps capable of working interchangeably with both old and new platforms.
Moving to double deck trains, even if you set aside the loading gauge issue (!) the key impact is the increased dwell time at stations whilst passengers entrain/detrain: that is going to decrease the line capacity far more than the increase in the number of seated passengers that can be carried. Page 63 of the document referenced above makes specific reference to this point.
At grouping, the railway companies found that a composite/universal English loading gauge had a height of 12’ 10”, whereas in Scotland this could be increased - at the apex - to 13’ 6”. I have no idea what the GWR had, but it is entirely feasible that theirs was more generous, and that the W6 “universal” loading gauge is lower, hence the problem with the Kings.
In short, the biggest single issue with our loading gauge is the reduced width below 43” above the rail, to accommodate our platforms. That’s not going to change. All of the other issues generally relate to putting square boxes on top of this (for containers).
No one would design an imaginary locomotive based on a requirement for, say, a broader gauge (larger inside cylinders, say) so what is the point in taking that approach to the loading gauge?
The simple fact is, double track trains would only be practical on a newly built dedicated railway, and would lack any form of interchangeability with the rest of the system, so may as well be built to a different track gauge. No amount of playing around with designs is going to change that.