All mobile networks can be very patchy and I think it's difficult to recommend one particular network, especially knowing (roughly) where you live as the geography around The Downs can push the signal from max to none in just a few paces.
For general use, I reckon I can offer a comparison as my work phone is Vodaphone, my last work phone was Orange, before Three we were on T-Mobile for about 5 years when I was working in Germany... I've used everything except O2.
I would say that Three is as good as the others in general, and better than them for the exact location of my house.
One thing that you
should find, though, is that you can get a "cooling off period" on any mobile contract. I think we had either 10 days or 14 days to return the phone for full credit if we found there was little or no signal where we needed it. Ask in the shop, though, something like that was a deal breaker for me and it might be for you too.