I have been with this company for nearly 20 years, back when they were called BT.
When I first bought a Mobile, the digital network wasn't really up and running, so I went for an analogue phone, and then changed over to a digital one a few years later. At the time BT gave the best coverage over the UK as a whole, slightly better than Vodaphone, which was the only other provider.
Be careful about being misled by some companies....Tesco for instance use O2 for their network.
In all my time with O2, I have never been let down, and have always changed phones every 2-3 years.
Beware that O2 are just about to be taken over by Sky, so perhaps things might change !
Don't forget that if you decide to go with another network, you can easily keep your number. Years ago, you had to get a new number, which was a pain in the arse to say the least.