Mike try Dyfedd steel, they are about the cheapest supplier I have found, they should deliver to North Wales.
I paid just under £100 for a sheet 2.5x1.25 of 6mm not too long ago.
The problem I can see you having is needing 2 and a half foot. We get tops made but we buy a complete sheet and its cut into 4. Which leaves you with 4 tops. Needing the extra 6 inches means it leaves you a piece of 1 foot 6 and thats not much use to many people.
how about having a chat with your local laser or plasma cutting firm, they might have an offcut they can trim up for you, and you don't want to be cutting 10mm plate yourself.