Serious question Hon Bookie, so a serious reply:
It could have been a route recce for an upcoming flypast. It could have been for a photo opportunity. And it could equally have been just a jolly.
If you were wondering if it had any more sinister connotations, I very much doubt it. The apache has limited uses over central London in the "finding the bad guys" vein!
Other aircraft do that job much better.
I personally have flown over the capital last year as part of a route recce, so I'm just guessing at the above possible reasons. Not being a rotary man, I couldn't say for sure, but hopefully one of our AAC brethren might be able to give you the definitive answer.