the standard ply board is 8' x 4' but some may be 2400 x 1200.
18mm birch ply should be easy to find, 24/25mm might have to be ordered.
Try phoning round to find a supplier prepared to cut for you.
You could try Avon Plywood (www.avonplywood.co.uk). Not used them myself but I know of other UK DIYer's who have and seem to be pleased with their service. They stock Birch Plywood in 18 and 24mm and they also offer CNC and cutting facilities.