According to the Independent Footwear Retailers Association - the answer is 8-and-a-half.

I've also spoken to Clark's, Britain's leading retailer, who confiirm the average is 'somewhere between 8 and 8-and-a-half' (but no, they don't make shoes in the size 8-and-a-quarter!). Put another way, the average is approx 266mm.

This figure is relatively static, but women's average shoes are growing: Clark's are producing many more women's shoes in size 8 and above than before.