Slightly related: As some HST sets kept two catering cars until sometime in the 80s on the East Coast for 'Pullman' type services, was there a (short) overlap where some HST sets could've operated both with two catering vehicles AND a TGS?
The paper order for CrossCountry (North East - South West services) was eighteen 2+7 sets, with a final top-up order for East Coast services of 4 2+8 sets (they originally wanted seven additional sets).
The first TGS (44000) was ordered vice the last TS (which would have been 42306) of set No. 14 (253041) of the second GWML sets, which should have been 253041. However, because the ECML second batch was only four vice the wanted seven, BR did a bit of jiggling and and extra TS was ordered as part of that second ECML batch to divert a GWML set to the ECML (the seventeen TS vehicles being 42306-42322).
Still with me? 253041 was, I believe, never delivered to the GW as intended and the vehicles were formed up into NE-SW sets. To boost capacity on the ECML the first five of the eighteen intended for the NE-SW sets went to the ECML as 2+8 sets, with the remaining thirteen making up NE-SW sets along with the vehicles from 253041 and those, on paper, ordered as ECML batch 2 to give eighteen NE-SW sets.
However, because the TGS was a new vehicle type the production run (44001-44090) were ordered instead of the TS vehicles for the eighteen NE-SW sets, and as they were inserted into 253001-253040/254001-254032 the 72 released TS vehicles were used to make up the five batch 2 ECML sets plus NE-SW sets, with 4xTS in a set with one of the remaining unallocated TGS vehicles.
With me still? The catering vehicles ordered as part of the NE-SW batch were all TRUBs - five went in the batch 2 ECML sets (254033-254037) and the remaining thirteen went in GWML batch 1 sets (253001-253027) releasing TRSBs for use in the NE-SW sets. The final four TRUBs (40353-40357), which on paper had been part of the four ECML batch 2 sets also went into GWML sets, as did TRUB 40335 that would have been part of the original GWML 253041.
Basically BR was quite clever and each HST batch after batch 1 saw vehicles ordered and used in other batches that allowed for reformations (move of the TRUKs to the ECML; addition of TGS to every set). Even with the 32 set order for the ECML (batch 1) the first twenty TS vehicles (42091-42110) were built first and sent to the GW to release the TRUKs. When the TGS vehicles came along these were the vehicles removed from each set and then used to make up the TS vehicles in five NE-SW; two of these were then disbanded in the mid-1980s and the TS vehicles (42100-42107) used to replace the TRUKs in sets 254012-254019 on the ECML!
HST formations have always been very fluid due to constant reformations to meet changing service needs for over forty years.