The original DfT plan in the DA was to consider retaining HST vs 222. The 800 series were out of favour because of the cost of extending the IEP contract. SDG had done an 800 timetable West Country cl.800 option for "The Great Cartographer" as part of the original IET contract discussions but the option didn't get to contract stage.
When the 222 proposal became a no-go (as everyone thought it would!), it became cl.802 vs HST and the 802 won hands down on cost and deliverability. The problem was a 9 car killed the Cornish half hourly scheme, the cost of going all 9 car was prohibitive and there wasn't the all year round demand to justify it. So to get the proposition to fly and preserve the Cornish half-hourly, portion working saved the day. You could always go from 5 to 9 at a later date, when the all year round demand justified it but you wouldn't be able to easily resurrect the Cornish half hourly.
The potential for a different interior design of the cl.802 caused the DfT a major panic. It is fair to say (and I was there) they would not have been authorized by the DfT if they differed internally from the 800 build. It was a very, very sensitive issue with them.
Going from 5 to 9 on the 802 fleet to make all the West Country workings 9 car isn't a big deal. You can do it with 48 more cars (24 x GU, 12 x motor only and 12 x Trailer) to do 12 sets but if you want some kind of bun truck you would probably do 14 sets (28 x Bun truck GU, 14 x motor only and 14 x Trailer). That would make all the 802 x 9 car fitted with a buffet. You then could keep 8 or 10 x 5 car 802 on the Bedwyns and ex 180 North Cots 5 car workings. Putting this stock on regional services to absorb 5 car sets is a possibility, Cardiff to Penzance is the most likely as 26m long vehicles on Pompey-Cardiff is not going to work, even if they could keep to time.
Altering 800 sets is currently seen as too difficult. The hoops of fire you have to go through to do even a minor change on the IEP TARA means that if you had to boost formations, you go to the 802 fleet first. Re-instating Ponsondane sidings at Penzance to enable 9 cars to be stabled down there is currently required if GWR went to all 9 car operation on the London-Plymouth/Penzance trains and that needs money. Is the DfT up for all of this? Frankly, no. A two year DA doesn't make it easy to finance and they are still nervous about the buffet issue. As they don't care much for what TOCs or user groups think, a lot of Tory MP's need to get on Grayling's (or his SPADs) case to make it all happen.