Just to clarify, it is not a 12v system (at least on the 777-300 I've just come off) it's 120v with a dual flat pin and round pin socket, ie a regular USA/European wall socket. Most laptop power supplies nowadays will operate 100-240v so it's just a case of making sure you can make the physical connection to the socket, and sounds like the FAs can facilitate adaptors for UK and other types.