I think it would be possible to revive a UK brand, but clearly developing a new engine is not cost effective. The obvious thing is to buy in an engine and build a UK frame. There are few engines available, The obvious one being Rotax as they are one of the few manufacturers that sell just engines. I have no idea if the Rotax is suitable for this application? They are sold for go-karts and I would think therefore have a different power delivery to that which a trials bike needs. I'd love to hear if anyone has any expertise in this area. Also I don't know if this meets the current Euro 4 needs? Even with the UK out the EU we will still need to comply with all the European standards if we are to sell into that market.
There is a good spread of technology there already with the Italian and Spanish bikes and Montesa Honda benefiting from the new EU-Japan deal. It would be a very tough market to enter, particularly as an outsider. That said the brands like BSA are very well known and could be a selling point, however you would first need to buy the brand from whoever owns it.
Basically we are reinventing the wheel and the way to go is to buy the Chinese bike!