During the last year or so, mobile connectivity has been pretty dire for me both in the flat and generally around the area. It seemed to work pretty well prior to that so I suspect it had something to do with the T-Mobile/Orange merger and that 3 had to remove service from a number of masts it shared with one (both?) of those providers.
I'd hold off signing a contract until you could borrow an unlocked phone (I'd lend you mine, but have just sold it...) and maybe get a PAYG SIM for a week or so to test your connection. A bit costlier for the short term, but will give you a better indication of what a longer contract would look like.
And for the record, I switched to O2 on the basis that colleagues extolled their virtues. Not in my case :-(