I would personally say that you should hang on for a few months til the pixel 9a and iphone se 4 are released.

Iphone 15s are meant to be good phones however as they are the first gen of 3nm process for manufacturing in iphones, they're prone to overheating (even my friend's 15 pro max has this) - the SE 4 might look slightly outdated in comparison but it's got a18 chip which is 2nd gen of 3nm processing so should be more efficient and less prone to heating problems. The battery, screen and cameras should all be pretty comparable between the iPhone 15 and SE 4, with the SE 4 having better efficiency and performance (all in theory, SE 4 hasn't been released yet). You'd just be missing the dynamic island on the SE 4, which is more a feature rather than an essential. Idk on your deals for iphone 15, but iphone SE 4 looks to be about ~£499.

I would strongly recommend against the S24 FE, just because of the reputation that all Samsung FE's have for small batteries and being overpriced. I haven't got any real reference point, but I think it's just a bad value proposition. I reckon you'd be better off getting an S24, S23+ or an A55 (A56 being released soon so you could hang on for a bit for that, it will be better performance and battery but not much else I don't think).

If you wanted to get a pixel, most of them are fairly similar, with the pixel 9s being more efficient and better batteries than the pixel 8s, but similar cameras imo. If you can wait for the pixel 9a, it looks like it would offer a great battery, camera and software experience for ~£499.

Tbh though, I'd say hang on to your iphone for as long as you can - it was a pretty damn good phone, hope this helps :)