Hi Red 5, and welcome to the board
Here is a pricing page for High Impact Polystyrene sheet from a company based in St. Albans that will also deliver and custom cut sheets. They also do ABS (oh, and here's a
description of ABS from wiki).
To be honest I've never used the company in the link so I'm unsure if they have a Minimum Order Quantity in place, but a phone call wouldn't hurt. I should point out that the prices don't include VAT (spits on pavement!) and orders below £60 are slapped with a £12.50 delivery charge

(sorry).
Alternatively, there is
Bayplastics based in North Shields, Tyne & Wear. Unfortunately their server leaves a slimy trail behind it and you may find you need another shave before the page loads up
If you do choose one of the above for the large sheets, would you mind reporting back here with your experiences? It would be appreciated
Anyway, hope the above helps and best of luck, that's quite a project!