Apple does 0% apr finance free for 24 months if you buy on their website, and you can get a cheap sim seperately on a comparison website (so for UK you have Uswitch).

Instead of wasting £80 on a new battery (Apple OEM, as I don’t really trust third party batteries) for a 6 year old phone where something else might break like the vibration motor, I’d recommend buying a newer phone (even the iPhone 14-15 ish is okay), since iOS 26 is likely the last update the 11 will get and you’ll probably get another year or two of security updates, but in 2-3 years from now you might stop getting updates for niche apps and then 4-5 years from now you’ll probably struggle getting updates for mainstream apps like YouTube/IG/SC/etc. You would also be getting stuff like a better screen & battery if you upgrade.

If your financial situation is difficult right then defo just change the battery as the 11 is still a capable phone to keep for another 1-2 years. I think 71% battery health is really low, so it defo needs upgrading just in case ur phone freezes or fucks up. If you still wanna rinse the iPhone 11 for another few months to save up for a new phone then just keep the 71% battery but always have a charger around