That's very likely. Any programmer with even an entry level position at the LSE would get head hunted by multiple agents in the first month. And the investment banks do pay £700/day for talent.
I know at least two guys who are getting paid closer to £1000 a day to work in London for a bank. The money is pretty much insane imho, and the guys tells me the 'talent' is just like anywhere else - ie most people are meerly compentent (but still getting paid £500 plus a day). Even graduates with zero experience are taken on at £300 a day or more.
He seems to better than competent, and so has managed to negotiate a much higher rate.