I don't think there is any real mast building at the moment, however from our UK wide research O2 are the best for 2G coverage (Brighton to Glasgow) with Vodafone a close second. 3G coverage is very variable but in the south east Vodafone proved the most stable for us with Orange/T-mobile the fastest when you can get it (and are standing still) but useless on the move.
It really is dependent on where exactly you are, in my house we get on Tmobile, no Orange, no 3, 2G Vodafone and 3G O2.
Brighton to Glasgow? and what about the other 197 miles north of Glasgow? Because I can tell you that if you come up to Aberdeen you'll see that O2 are TERRIBLE (2G and 3G)
I use two phones on a day to day bases (one for work and one for me) My phone is on 3 and the work phone, O2. I get better 3G coverage on 3 than I do 2G coverage on O2.
Very rarely works properly, always cuts out, very slow oversubscribed data and shocking customer service. Why so many people stick with them is beyond me.
I pay £35/month to 3 for (virtually) unlimited everything. they never slow me down, no matter what i download (avg. ~5GB/month) and the speed is fantastic! My most recent speedtest on my iphone is 8.92mbps down, 4.14mbps up and a ping time of 77ms.
Not bad for a 3G network if you as me, a lot of LTE dosent touch that speed
Getting about 15mb down and 900k up on Sky DSL, not bad when it's free! Can't wait for FTTC (30-6-12)
Disclaimer: I work for sky, but have no involvement AT ALL with broadband, so dont ask!