The first aid sign is probably temporary and associated with the London marathon.
Signs etc usually start go up about 4 weeks or so before the day of the marathon.
It is probably indicating the first aid post or assembly RV point.
The marathon is normally run in the first half of April.
Forgot to add that it takes several weeks for all the signs to be removed after the marathon.
If I am correct this would confirm a time frame from roughly start of March to end of April plus or minus a small margin.
Getting all the signs etc placed and then removed must be a logistical nightmare and it is not beyond the realm for one or two to get missed during removal phase.
The marathon was run on Sunday the 13th April 2014.
Lots of bike couriers in London so it is possible.