You can fit any rear motor as long as it has hall sensors, which is most of them. That's the easy bit. Now it gets more complicated.
If you have cassette gears (8 speed or more), you need a cassette motor (spline for cassette). If you have 7-speed or less, you need a freewheel motor (thread for free-wheel).
You need a motor with a nine-pin connector unless you don't mind cutting off the connectors and soldering the nine wires together.
You need a motor with an internal speed sensor unless you don't mind hacking the wiring at the controller end to splice in an external speed sensor.
I don't know anybody in the UK that sells a motor wheel on its own, so you have to get one from China, which is no problem, but shipping is expensive for complete wheels.
If I were in your position, I'd buy a 260 rpm Q100H as bare motor and get it built into a rim in the UK. that will cost £100 for the motor, maybe £15 duty, £30 for the rim and spokes and about £30 for building unless you can do it yourself. There's a 201 rpm version as well that will have better climbing efficiency, but max speed is only about 15 mph.
https://bmsbattery.com/ebike-kit/631-q100h-36v350w-rear-driving-ebike-hub-motor-ebike-kit.html?search_query=q100h&results=5#/327-rpm-260
If you have cassette gears, it's the Q100C, which only comes in 201 rpm flavour, so slower than the Q100H.
On Aliexpress, there are all sorts of weird and wonderful motors that would do the job. You just have to search through all the listings.