The first thing to decide is which providers are good in your area.
There are essentially 4 networks in the UK now.
EE, which includes legacy T Mobile and Orange accounts.
O2
Vodafone and
3.
Each network has what are called MVNOs (Mobile Virtual Network Operators) that "piggy-back" off the networks.
Eg. Virgin and Asda Mobile piggy-back off EE.
Tesco Mobile piggy-back off O2.
Talk Mobile piggy-back off Vodafone.
ID mobile piggy-back off 3.
There are plenty of other MVNOs.
They generally have much cheaper charges than the networks.
The exception is 3, who offer what they call a 321 SIM. 3p per minute calls, 2p per text, 1p per MB of data.
Every network has a limit on the number of chargeable event-free days before being disconnected. Usually 180 days, but it depends on the network.
Just send a text every so often.
Whatever you do, be wary of some non-contract SIM cards that are set up so your top-up expires at the end of 30 days. You need a SIM like 3's PAYG or Asda PAYG, where your top up will last indefinitely, as long as you make a chargeable call or send a text every so often.