Ah! You have stumbled upon the next topic in the Brewing Water Series...London. Assuming you are an AHA member, you would have read about Burton water a few months ago in Zymurgy. Certainly a high sulfate content there.
London water is very interesting in that there are actually two unique water profiles. The water from the upper River Thames and River Lee are reasonably suited for pale ales and bitters, while the water from the chalk aquifer and the tidal Thames is more suited to porters. In either water, the sulfate content is much lower than Burton water.
You will have to forgive me, but you'll have to wait another month for the next issue of Zymurgy to get the particulars about London water. Yet another reason to be an AHA member.