Not sure where you are in the SE but I have never not had a 3G connection wherever I go in the SE, it's the very reason I have just signed another 24 month contract with them.
I tried all the other providers which were okay but it was hit and miss with signal coverage and Three was my last ditch attempt at getting something reasonably decent and even then I didn't hold out much hope...never looked back since, not only that their offers dump all over the other networks!
The only place I didn't get a very good signal was in my house but that was the same with all the other networks I tried, the only difference being was when I explained this to Three they sent me a Home Signal box and I now have 5 bars wherever I am in the house.
So strange. In Oxfordshire i very rarely see 3G. Often its the 'o' symbol of doom. Most of the way to London (High Wycombe) i can find myself without signal at all. I thought it was the iPhone 4 antenna thing, but its a similar story with the 5. London is at least covered fine. However, just went to France and never once found myself without 3G on the SFR network. So its probably not the phones fault. Confused