Unfortunately, although the US NES Carts look identical to those produced in the UK, they will not run on a PAL System without a little intervention. Getting the NES modified is one option if you can find someone to do it, but there are also a few adaptors available. Probably the easiest one to to get hold of would the Gamekey Adaptor by Horeleg. This comes in two flavours – the first design requires a UK NES Cartridge to boot the US Cart while the Gamekey II has a more compact design and also doesn’t require a boot cartridge.
To my knowledge, there are no retailers still selling these adaptors but they can be found on Ebay.