Generally your best bet for finding Shonen Jump in the UK is to look for a comic book shop. For example "Travelling Man" in Bristol has the three most recent ones (though that may not be as beneficial for you) and I do believe there are other Travelling man branches throughout the country. Smaller comic book shops may also be able to provide you with it.
In my humble opinion I feel your best bet is to find a local comic book store, big or small, and ask if it would be possible for them to get it for you. I myself used to do a simular system for getting ahold of Ultimate Spider-Man. Alternatively you can do what I do - wait for the graphic novels to be released and buy them from either and online shop like Amazon.co.uk, or from a comic book shop and/or some book shops like Borders.