The 180s were of course originally ordered by Great Western Trains (prior to the FirstGroup takeover of the original management 'buyout') to run South Wales services with 10-cars running to Cardiff and dividing with 5 continuing to Swansea (and occasionally beyond). There was also I believe a plan at one point for a follow-on order for North Western Trains/First NorthWestern for services from North Wales/Blackpool/others to London.
The rather severe 'teething problems' with the initial 180s (and of course pretty much all other early '3rd generation' Alstom units!) put paid to all that (and led to an extra summer of 47/8s and Mk2s running to Devon and Cornwall!) and the 180s eventually found limited use on Thames Valley and Exeter semi-fasts before being sent off-lease and picked up by the open access operators (and Northern of course!).
As for the location of first class, I imagine there was an initial reason for not using a driving car, perhaps to do with ratios of 1st to standard and the problem of crumple zones in the driving cars.
One advantage of this layout was that the rear car in each set was the isolated standard car and many passengers at Paddington, being used to HSTs, assumed this was 1st class (the antimacassars saying 'First' probably helped too!

). This meant on arrival at Reading at 7pm I had the pick of the seats in there for the run to Taunton having travelled down from Ormskirk via Preston and Birmingham, in stark contrast to the sardine-like conditions in the rest of the set!
