Hi there
To help people work out the differences, here is how 6700, 6700 XT and 6750 XT differ:
6600 / 6700 / 6700XT / 6750XT
7nm / 7nm / 7nm / 7nm
2491MHz / 2495MHz / 2581MHz / 2630MHz - Boost clocks
1792 / 2304 / 2560 / 2560 - Stream Processors
128-bit / 160-bit / 192-bit / 192-bit - Memory interface
16GBPs / 16GBPs / 16GBPs / 18GBPs - Memory speed
8GB / 10GB / 12GB / 12GB - VRAM
The 6750 XT is just an overclocked 6700 XT, there are no major changes, just a slight boost to core speed and memory clock.
The 6700 has more stream processors than a 6650 XT and a larger memory interface and more memory capacity with slightly lower clock speeds.
In short it is fair to summarise that a 6700 will match or slightly beat a 6650 XT at 1080P resolution and pull further ahead at 1440P resolution due to the 160-bit memory interface and extra 2GB of VRAM. As such a 6700 is a real bargain as it is priced below the current 6650 XT whilst board partners clear stock of 6700.
A 6700 XT is of course faster, particular at 1440P but it is not £100 worth faster in my personal view.
I'd say with a £400 budget the RX 6700 is currently the best buy on the market as it is an exceptionally strong 1080P and 1440P graphics card. Of course the 6700 XT and 6750 XT are faster but are they worth the extra outlay?
Also remember you can now buy a 6800 from £569 which has a whopping 3840 stream processors, 16GB VRAM on a 256-bit interface, as such is a considerable leap in performance even over a 6750 XT especially at 1440P and above resolutions.