After getting annoyed with the reliability of modern phone battery life, with both Apple and LG not lasting as long as their respective contracts, have for a couple of years now bought an unlocked Android phone from Amazon for around £100.
I have happily bought a Chinese make called Cubot from Amazon and have not regretted it. Only issue being the GPS is a little inaccurate and the camera could be better. But technically it runs all android apps available. There are some superb bargains about, albeit with possible Uk support issues. Google “Best Buy Chinese phones”. The quality is getting much better.
For a little more you could look at the Motorola range. A G7 play is around £130 from Amazon.
Samsung also do a range of sub £150 phones if you want to stick to a more reliable phone brand. A J5 has a price of about £140.
As all these phones are bought without a contract and are unlocked, you simply insert you old sim card and off you go. Although modern phones may take a different sized SIM card if your current one isn’t compatible it’s very easy to get a new one from your current provider free of charge.