Unfortunately not sure what your options in the UK are, but I previously just researched the heck out of tires at tirerack. Most recently I used what was available at Costco, and did a cross check with tirerack with reviews and prices of what I could tell where comparable tires.

I know you didn't want to get into it, but there's X5's out there with non run-flats on them that came that way from the factory. So don't worry about going against what the engineers designed it for. If anything most people feel ride quality improves moving away from run-flats.

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'08 X5 4.8i - 196k
'15 i3 REX - 95k - Blown AC compressor

'98 323is - 170k - Garaged
'94 325i - 208k - Sold