I'm in a similar dilemma at the moment.
I am with Asda mobile, who are a MVNO (Mobile Virtual Network Operator) piggy-backing off Vodafone. Voda's signal in Stockport is carp. They are actually moving their whole network in April to EE... which is equally carp where I live in SK7. Mrs Secure has a Virgin SIM. They too are an MVNO, piggy backing off T Mobile, who have joined up with Orange. Their signal is carp here, as is O2's. It's no good going with Sainsbury (they are MVNO too, piggy-backing off Voda) or GiffGaff (MVNO, O2).
So, that leaves 3 for me.
What I would suggest, WS, is the cheapest option is to work out which areas you use the phone the most. Then ask everybody you know in those localities what mobile operator they use and what the signal is like.
When you've pinpointed a couple of hopefuls, buy a bog standard PAYG SIM card (or better still, blag one FOC) and load it with a fiver of call time. Then use it in a spare handset (If you've not got one, I can send you one, no bother) to see what the coverage is like and how you get on.
This is exactly what I am doing with 3 at the minute. They've got 321 SIM: 3p for calls, 2p for texts, 1p per MB data PAYG. I'm not using the data and I'm still on my first £5 having topped up a couple of months ago!
I can send and receive both texts and voice calls from inside the house every time: something I cannot do with any other carrier.
Give it a go!