This is mostly about the fact that there is a holy crapload of RF stuff going on in the area - networks, wireless TV camera links, telemetry, wireless mics and in-ear monitors/talkback, walkie-talkies etc. so they need to be able to find anything causing interference that might disrupt operations. The odd random person sharing a 3g connection to save BT's rip-off charges will be the least of their problems.
The antenna in that pic looks like a wideband one - if they were just looking for wifi they's use a single band one.
As well as mobile receivers there are some fixed sensors to get rough positioning as it's a huge area to walk around- I'm guessing these are remotely operable spectrum analysers or simliar.
There are some reports on all the spectrum requirements they envisage and there is all sorts of stuff there.
http://stakeholders.ofcom.org.uk/binaries/consultations/london2012/statement/statement.pdfhttp://stakeholders.ofcom.org.uk/binaries/consultations/london2012/statement/london2010Update.pdfSkimming the second, looks like 2.6GHz is being used as a dedicated band, so interference from 2.45G wifi is a distinct possibility.
When I was working there we were told that any wireless networking gear we installed had to be on 5.8GHz.
of course "wifi Police" makes a much better headline