Contract phones can often work out dearer than buying a SIM-free phone outright and then using recurring pay-as-you-go (PAYG) bundles.
If you can find a SIM-free phone you like, the most popular providers of PAYG bundles among AB member seem to be GiffGaff
https://www.giffgaff.com/sim-only-deals
and Asda Mobile (which I use):
https://mobile.asda.com/
Smarty (which is relatively new) is providing them with plenty of competition though:
https://smarty.co.uk/sim-only
(For comparison, you only get 2 GB of data with Tesco Mobile on a similar plan. My suggestions above are all far more generous:
https://www.tescomobile.com/shop/sim-only-deals/sim-only-contracts ).
The best SIM-free phone available at the moment (in my opinion) if you don't want to pay mega-bucks for one, is the Samsung Galaxy A12, at £150 from Argos:
https://www.argos.co.uk/product/8886745
It only came out in February of this year, so it uses the latest version of Android. It's got plenty of memory and a battery that lasts about a week without recharging. (It's what I bought myself just a few months ago). Apart from not having 5G capability (which very few people actually need anyway, in my opinion), it's very little different from the Galaxy S21, which costs between £820 and £1200, so that's quite a saving! [NB: You might find the Galaxy A12 a few pounds cheaper elsewhere but such offers are all for phones which are locked to relatively expensive service providers. Avoid!].
If you're on a really tight budget though, consider Alcatel phones as they have some quite cheap models which are reasonably good relative to what you pay for them.
If you're determined to buy a phone which is locked to a particular network (although I don't advise doing so), make sure that it's not only a decent phone but that it uses an inexpensive network as well. With those two factors in mind, the Nokia 3.4, locked to Asda Mobile, is well worth considering at £105:
https://groceries.asda.com/product/mobile-phones/asda-mobile-nokia-3-4-charcoal-mobile-phone/1000304248699