Sorry to be pedantic about this, but railcar no 1 was not used on the Cardif - Birmingham service.
Railcar no 1 was the original, and the only one with just one engine. It was fitted-out with a bus type interior, with about 70 seats. It seems to have spent most of the time in the Thames Valley, I assume because of less power than the later railcars.
Railcars 2, 3 and 4 were the ones fitted-out with a buffet counter, and 2 toilets. They were used initially on the Cardif - Birmingham route via Gloucester.
Railcars 5 to 16 were fitted out with bus type interiors, with 10 to 12 having a single toilet.
I agree that 5 to 16 would have seemed to be the obvious choice to model, but with the opportunity to laser-scan no 4 at Swindon makes the task of replicating the subtle complex curves of these vehicles a bit easier.