I did this a couple of years ago with a company operating out of Elstree. The tour I did included flying down the Thames and an overflight of LHR
A couple of issues from a photographic point of view - these are tour flights, not photo flights ... no open doors or windows. Unless you bag the front seat, you will be jammed in with others in the back, and your ability to get into a good position for a shot is restricted. Can't recall the type of helicopter off hand, but all the glass had a degree of curvature meaning you had to have the lens square on to avoid distortion, reflections.
Having said all that, decent shots were possible, including over LHR
. I used a 70-200 which was a bit short - though there wasn't much room for anything larger. You'd be OK
with a 100-400 in the front, but not in the back. Note that the LHR
bit was just an overflight, no hanging around, so you'd be lucky to be at the right point just as an aircraft landed or departed - in my case, all the "best" shots seemed to require pointing the camera at far too extreme an angle to get a decent result.
Overall verdict, well despite the fact the weather was poor, it was a very enjoyable experience - but as an opportunity for good
photography, pretty limited. You will doubtless get a shot of Big Ben etc. but I'm tempted to say you would be better leaving the camera at home, enjoy the flight, and buy a postcard instead
Colin K. Work, Pixstel