I know, it's been taken by every man & his dog but while I was in London recently I just had to take my own!
For anyone who is interested it's taken fron an opening on the first floor level. Oh and another thing, to get as much as this in the shot you need a wide angle lens. I used my Sigma 10-20 at 10mm. I only wished I had my tripod with me to have a bit of movement of the people. Apparently if you ask at the desk they will allow tripod use.