yea keep the money in the US because the global economy would be horrendously upset by you spending 18 US Dollars in Great Britain.
Urban express is the NU50's name in the US, Runaway was its name overseas. NU50=NU50. You can get specific NU50 gaskets, OR if you just need topend gaskets you can use ones from the honda hobbit (PA50) as well. including the intake gasket. The NX50 has the same gaskets the whole way through.
OR even cheaper you can use hondabond/yamabond/permatex sealer for the case halves. You can also make your own gaskets out of gasket paper, trace and cut, with the exception of the head gasket which you can reuse or omit entirely.