I always find Micheldever Tyres are best for any performance tyre and they usually have quite a choice, even in exotic sizes. Micheldever Tyre & Auto Services
I believe they are the largest independent tyre dealer in the UK and, if you're in Slough, it's not too far a run. Beware, there is always a big queue, especially at peak times, for their 'drive through' fitting service but there is a comfortable waiting area they have just extended.
As has been said, you may consider ditching the run-flats. There is plenty of discussion about it on this forum. I've gone for Vredestein Ultrac 17" non-run-flats (not available from Micheldever, though) on my 2004 Cooper; very smooth and great wet grip. Topically, the Ultrac is a summer tyre, I'm running Michelin Alpins at the moment for winter... I must be the only person in the south of England with winter tyres! They also do a run-flat - Quatrac 3. That is new and I've not tried them; I ran Goodyear run-flats and found them better than the others, especially Pirelli, which I found horrible.
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