SHJ, there was a Partenavia tooling around the Croydon area at lowish level on Saturday (I'd have guessed it was at 1000'), and a photographer I know of through work was planning on doing some aerial photography around London yesterday - he usually uses a Seneca with the door off and whenever I've seen him at work, the aircraft is at least 1000'.
When I got up early yesterday, a C406 (G-FIND?) was just finishing an impressively neat series of NE/SW runs across Gatwick at about 10000', each perhaps 15 miles long - quite a grid formed by the trace on FR24.
Once saw a banner towing C172 orbiting Croydon at well under 1000' - our offices are around 180' off the ground and at times it looked to be nearly level with us...