So long as you are using it a flat hard surface it will not break, most hardwear stores have it, Homebase, B&Q, Focus, they will probably have it in the sizes you want. Easy to clean with bacterial soap and should not blunten knifes any more than wooden boards would.
They are now saying that as soft plastic boards sort of 'heal' they can trap bacteria in them which can be let out the next time the old scar is cut through. As has already been said, this is leading to wooden boards becoming more popular.
Having said that, I get plastic boards from Ikea, they are so cheap as to be almost disposable.
You should be able to get PE flat sheet from engineering supplies.
I never understood what is meant by perspex, if polystyrene is meant,
that is brittle as hell and can crack easily. If one means Polycarbonate that could be useable. PE is better though. Polypropylen would do also.
Plastic blenders have already made plastics with antibacterial additives.
I think that most of the domestic butchers blocks that you see are maple, but they should always be made up of planks on end. The cheaper blocks seem to be planks across the block and these are supposed no good.
Hornbeams aren't that common i certainly dont work on them very often and when i do they are never big. the wood is very hard though and i imagine that they would make an excellent chopping block, i believe it used to be called iron wood for obvious reasons.