My recommendation is to buy a SIM-free phone and then to find a service provider offering pay-as-you-go bundles that will suit your needs.
Pay-as-you-go used to work out a lot more expensive than contracts but it's often at least as cheap, and quite possibly cheaper, these days. Since you're not stuck with a fixed-term contract, you can easily change the type of bundle you use at any time. (If you're happy to stick with the same bundle though you don't have to remember to renew it each month. You can set things up so that it automatically renews itself).
I use Asda Mobile's £12 bundles but, for the amount I use my phone, I could almost certainly get by if I reduced my payment to £10 per month, or possibly even to only £7. Details here:
https://mobile.asda.com/bundles
(Asda Mobile has outstanding customer service. Calls are answered in seconds by helpful and knowledgeable staff in the UK. It was named a 'recommended provider' by Which last year).
The other PAYG service that's very popular among ABers seems to be GiffGaff. Their prices are here:
https://www.giffgaff.com/sim-only-plans
[If you're worried about signal strength at your location (or where you travel to), Carphone Warehouse's iD service uses Three's masts. GiffGaff uses those operated by O2. Asda Mobile uses EE's service].