Only tennis magazine I can think of is Ace Tennis magazine (and the British tennis magazine that comes with it).
For the tennis websites, the LTA website is probably the best place to start as it will have a load of general information, and there will be links to players sites and stuff.
As a general forum, this is probably the best place for discussion surrounding British tennis.
As for general GB tennis sites, as FD says, this is clearly the best unofficial one (even if we do say so ourselves!) simply because it's such a co-operative effort from so many committed people that there's always something new and interesting on it.
There's also http://www.britishtennis.com/ which seems more commercial (i.e. it tries to sell you stuff) with less content, BTZ at http://www.britishtenniszone.net/, the original site for enthusiasts who supposrted Tigger and Greg but could see past them too, though it's probably fair to say that is has been superseded by this site nowadays and many of its members post here.
There is also http://www.centrecourthavoc.com/, a mainly British offshoot of the BBC forums made up of people who got fed up of the tripe on there - they're very good at predictions and I mustn't forget GB Tennis girls http://www.gbtennisgirls.com/, set up by some female former GB players which concentrates exclusively on the women's side.