Bob
Because unlike the US, nothing is allowed in there without a working Transponder and you don't see a British Army defender (or any other small aircraft) mixing it with the heavies (South London is the normal approach path into Heathrow), the airspace is covered with both primary and secondary radar which can be monitored on the internet via Heathrow Airports own web site. The only way such surveillance would work would be with 24 hour coverage, which isn't happening as there is nothing orbiting southern London at the minute. The other thing is the report comes from the Daily Fail, a UK newspaper not known of its reporting of the actual truth on anything. Some FBI guys and a Brit in the back of a van parked not far from the target locations with some form of EM radiation monitoring kit is far more likely, If the British are not doing it themselves by tapping the landlines.