Please remember that any mention of competitors, hinting at competitors or offering to provide details of competitors will result in an account suspension. The full rules can be found under the 'Terms and Rules' link in the bottom right corner of your screen. Just don't mention competitors in any way, shape or form and you'll be OK.
£300 brand new is a good price for a 6750 XT. I'd personally pay the extra for the 6800, especially since you have a 7600X, but the 6750 XT is decent enough for 1440p.As looking for one at round £300, that will do the job well.
Open to looking at second hand cards. I will see what’s about for the 6800 and 6800xt. As the 6750xt I had eye on is new or can be had around £250-£280 second hand.