My local timber merchant stocks the Phenolic ply. £50 a sheet. I’ll probably need three.
Theres phenolic and phenolic – see what Meyers have, from memory they’ve three or four ply types with phenolic facings – birch, hardwood, gaboon (marine type), poplar, etc so prices should vary a bit. Mind you you wouldn’t get much change out of £40/sheet for regular plywood.
If there is plenty of support, and the loads are well spread then another option is Stokboard – sheets of recycled plastic, not very stiff, but very durable and it doesn’t rot. Might get it from horsey/farmy type suppliers
Service Metals over in Cumbernauld sell the various bits and bobs for building truck bodies and will have flooring materials too (under transport products – look under ‘f’ for floor planks and ‘t’ for timber floors