sintra is actually a brand, not a type of plastic, Much like what COCA COLA is to "soda". Everyone else tries to replicate sintra, but in my experience, has always fallen short, much like RC is to Coke. You know both are colas, but one is obviously better. Your best bet would be to call Sintra or a distribution center for Sintra and ask them if they ship or if there are any UK distribution centers.
Sintra/Foamex/Foamed PVC is not to hard to work with, it needs to be heated up to be molded (a hot gun works well, but be carefull not to over-heat it!) And wear a a good quality dust mask when cutting, the dust is nasty.