Tricky. You bought a product from a US firm (Pi-Top) through a US website (Kickstarter). So your rights are those under US law, meaning that any legal action would have to be through US courts under the relevant State laws, not Federal ones.
I note, however, that there is UK address on the Pi-Top website, so you could try sending a letter by registered mail, threatening either legal action or (possibly more effective) adverse publicity via the technical press. The address given is:
PI–TOP
499 Hackney Rd
London
E2 9ED
Don't expect to be contacting a massive office by using that address though. Googling shows that it's actually the address of Hackney Coffee Company!
You might get more joy through buying a copy of Computer Active magazine and contacting their guy who specialises in sorting out complaint issues with companies. (They often respond quickly if they're threatened with poor publicity).