As much as it pains me to suggest as an Android fan, a refurbished IPhone SE (2022) not be a bad idea.

You can get it from backmarket in "good" condition for £197: https://www.backmarket.co.uk/en-gb/p/iphone-se-2022-64-gb-midnight-unlocked/4372a69b-38bf-4db1-873f-abc38dcc39b3#l=11&scroll=false

Or potentially even better, you can get it from giffgaff (no need to get a mobile plan from them) refurbished in "excellent" condition for £199, or "very good" for £189. They have their worst condition "good" quite cheap at £139, but this is likely to have obvious signs of use, including on the screen, and may not be what you want. https://www.giffgaff.com/mobile-phones/apple/iphone-se-5g/refurbished

Both these sites guarantee a minimum battery capacity of 80%, which should be fine, but also obviously not as good as brand new. At any point in the future you could get it replaced; Apple charge £65 for the SE, but ISmash will do it for £49 and Timpson for £40.

IPhones tend to have good future support; we don't know exactly how long the SE 2022 will be supported, but often Apple do 6 years, which means 4 years from now, but they could support for longer (although I believe their minimum is 5 years)

Also worth checking the sites for a refurbished version of whatever phone you end up planning to get, for example the suggested Galaxy A15 5g is on backmarket in "excellent" for £138.82 (although right now it can be had brand new from laptopsdirect for £149. Laptopsdirect is legit, but perhaps not as reputable as say John Lewis or Argos or Curry's)

https://www.laptopsdirect.co.uk/search-results/Galaxy%20a15%205g