My Nokia 2G handsets from the late 2000s still have good battery runtime and spare batteries are easy to come by. You can carry a spare battery instead of a charger. A week of standby is typical. They sell for around £10 on ebay and similar sites.
A Blackberry from the early 2010s will give similar functionality (calls and texts) but gives a much better text message experience due to a QWERTY keyboard. You'd get a standby time of 3 days and again the battery is removable so you can carry a spare. The models from around 2013 onwards also make usable music players, being compatible with multiple audio formats (including FLAC). You can buy these handsets for less than £10.
These are just starting to find a market as 'digital detox' phones for the smartphone generation, but prices haven't risen much as supply is plentiful - almost everyone has a spare phone in a drawer or in the loft.
As 3G networks are dismantled, any 3G handest will revert to the 2G network for calls, but that was good enough then, and it's good enough now IME, especially if this is being kept for occasional use.
Best PAYG deal I have found is 1p mobile, which requires £10 top up per quarter or £30 per year, on a direct debit. Texts cost 1p each to send, voice calls are 1p per minute and the credit rolls over. It runs on the EE network so will work for phones that have an Orange start-up screen or a T-Mobile one.
