Jailbreaking is illegal in some small European nations where there's some obscure law about data integrity; it is legal everywhere else (you bought the device, you should be able to do whatever the heck you want with it), except voiding warranty.

I'm not sure about any VoIP providers that offers inbound numbers for UK... I am in Canada, and I'm having a hard time finding one that works for my province, too. Most of them providers seem to only target USA, which is quite annoying for people like us who's not in USA :P