PAYG (virtual networks usually piggyback O2, ee or vodafone.)
Most supermarkets have a range of payg sims that usually cost £1, but make sure they say payg on the packaging.
Also make sure the credit rolls over and does not require a monthly payment.
They can be topped up using vouchers or with a phone via a credit card,or via a top up card.
The big players can have costly call rate charges, so supermarkets or other providers are cheaper, and most shops only offer contracts.
The payg phones do need to be used every 90 days or the sim will de-activate, I had that happen myself with giffgaff.
I was with asda myself previously but they switched from ee to vodafone, check the customer reviews on trustpilot.
Some contracts can be as cheap as £5/month and an alternative to payg.
Customer service can be patchy with some payg services, and although still available can be hard to find.
If calls only are required top up with credit only if data not needed, then do not choose a bundle.
Shame virgin have dropped payg, but it is still available with giffgaff, tesco, asda, lycamobile, vodfaone and many others if you google payg, but some companies no longer make it easy to get payg and not easy to set up.
The call costs vary a lot and information on payg is not easy to find now, it is getting harder to find or can be costly with the bigger operators.
I use giffgaff, tesco as main phones, (lycamobile as a backup phone).
Giffgaff payg is good,but is "online only" with no help phone line but works well, tesco has a good uk helpline and a light user tarriff if you ask for that and I am finding it good, but avoid buying packs/monthly costs, Asda is cheap but the calll centres are outside of the uk now via vodafone.
Always check the customer reviews on trustpilot, and there are cheap contracts now with companies such as lebara as an alternative to payg, I have not tried that, I still prefer payg which means you only pay for what you use. The 90 days of inactivity with payg is a nuisance if you do not make regular calls,the phone will stop working and you loose credit, I usually top up with as little as £5 or £10, some have a minimum of £10.