I haven’t checked the price of City Electrical Factors sheets but at 1.2M x 0.6M (4Ft x 2Ft) I think they’ll be quite costly and larger than most of us would need.
I mostly use GFG Plastics in Hull.
I’ve had several sheets of SRBP from them in black and brown in various thicknesses and sizes, and Acetal rod too. Their main business is making water storage tanks for the caravan industry, but they also supply a wide range of plastics in sheet and rod including SRBP in a wide range of thicknesses at sizes which suit our needs. They don’t have a counter service so though they’re less than five miles from me, I have to order via their eBay shop:
https://www.ebay.co.uk/str/gfgplasticsltd
The other firm I’ve used and found good is Trent Plastics.