Well the 'Never-wazza' Gresley 4-6-0 locomotive has arrived and the quality in the detail, valve gear and braking rods are perhaps the best that I have ever seen on a kit built locomotive. After the conversations that has stemmed from my post I have decided to draft a plausible Class Name and History for this 'might-have-been' class - I can only credit those who have contributed to the conversation (I have borrowed/stolen your ideas).
GNR 'I' Class 4-6-0
Looking at the GNR Classifications - the GNR never had a classification for a 4-6-0 as they never had a 4-6-0 class, however listed in the Classification Archives - The Letter 'I' was listed but not used; therefore the 'I' Class will be the allocated letter for a 4-6-0 configuration.
As stated by DaveCBroad:
"Between 1900 and 1914 all the main line railways had an ever increasing demand for fast freight trains. Some addressed it, the GCR, Caledonian, and especially LNWR with powerful fast impressive small-ish wheeled 4-6-0 locos specifically designed for the purpose. Others well didn't. The Midland stuck their heads in the sand and prayed it would go away, The GNR built 0-6-0s with 68" driving wheels and also used 4-4-0s and 4-4-2s. Neither really got the job done effectively so Herbert "Nigel" Gresley designed the 68" wheel K1 2-6-0, something of a gutless wonder, and then bigged up its boiler to make the very successful K2. This was still on the small side so he enlarged it...Cue your 4-6-0 because it looks like an elongated K2, a 2 cyl 4-6-0".
The model has 26.6mm dia. wheels (6ft 8in at OO Gauge) meaning that this was for fast long distance running, either fast/semi-fast passenger working or express fitted freights (fish). In fact the wheel diameter is the same as on the A1/ A3 and even A4 locomotives and also the B17 locomotives.
The GNR 'I' 4-6-0 would most likely have been the intermediate step between the mixed traffic K2 and the Express A1/A3. The GNR 'I' has 6ft 8in and certainly would have rivalled the later Thompson B1, but with a more enclosed Cab and larger 4200 Gallon tender, the later B1's would have larger coal and water capacity and more comfortable for crews and the smaller 6ft 2in dia. wheels of the B1 would have made it more suitable for freight workings. The B17, A1/A3 and A4 locomotives all had 3 cyl and were more suited to smoother running at speed. The larger tenders of the B17, A1/A3 and A4 - and there larger tenders made them more suitable for long distance running.
The LNER numbering of GNR locomotives was 3001-4770 (3000 added to original number), however the 1946 LNER Numbering designated numbering between 1000-1999 for six-coupled passenger and mixed traffic tender locomotives (combining 4-6-0, 2-6-0 and 2-6-2 configurations), so the GNR 'I' 4-6-0 would have fallen into this category.
The LNER classification for a 4-6-0 was designated as 'B' so the GNR 'I' would have been classified as 'LNER B?' - Therefore to keep it simple the GNR 'I' 4-6-0 will be classed as an LNER B 4-6-0 locomotive. But as the Thompson B1 commenced the 1000-1999 category, the likely numbering of the GNR 'I' would have been immediately after B1's, but the B16's took that place, leaving a gap between the B16's (61410-61478) and the B12's (61500-61580). The B18's were number 1479 and 1480 (1946 Numbering), the B4's were numbered 1481 to 1489, the B19's were numbered 1490 to 1493 (1946 Numbering) and the B3's were numbered 1494 to 1499. A suitable Number and BR Number needs to be allocated to Locomotive.
As with the a lot of the earlier locomotive classes being scrapped in the late 1940's and early 1950's, the lesser engines would have succumbed to the cutters torch, and as the more predominant classes extended there sphere of influence. The GNR 'I' / LNER 'B' would have either been moved further north 'my take' or scrapped very early in the 1950's when the need for older engines were surplus to the requirement, with newer locomotives from the Standardisation programme more evident.
I have attached images of the GNR 'I' and the GNR 'H3' LNER K2 as a comparison.