I have no special insight into the London market, but the fact that it hasn't happened after all these years of huge differences probably indicates that the arbitrage opportunity is not as large as it appears.
It seems like a mistake to measure productivity through salary. A baker in India will make a fraction of a baker in the EU, even if they're making as many breads or more per day.
It may simply be that the the same product is worth less when developed in London rather than SF, due to relative availability of VC, market access restrictions, cultural approaches to monetary compensation or any number of other reasons that have nothing to do with the quality of the code.