Hi Leo,
 
I am about 100% certain that you will find your bees are working on
Indian Balsam (Impatiens Glandulifera).
 
I am also a London beekeeper, and although it is not growing in my
immediate area it is growing prolifically out to the west of Londond
where my father has his apiaries. I have just come back from a weekend
going through his bees with the bee inspector doing the routine check
for EFB (all clear thank God) and we were seeing the bees going into all
his hives, exactly as you describe. His apiares are near the River Mole
and there is balsam everywhere at this time of year.
 
If you don't know it it is a tall plant, about a meter tall, with pink
to white 'slipper like' flowers and seed heads that pop when you touch
them. If you find it you will see that the pollen is so positioned that
the bees get covered with the stuff and can't clean off that particular
area which is especially covered.
 
We have noticed over in his colonies this year, a large quantity of deep
carnelian red honey that tastes very strong and, I think, rather
perfumed in a 'damp basement and josticks' sort of way. The flowers have
the same sort of smell en masse, hence our current opinion that it is
the balsam.
 
Quite a few beekeepers in the area are also reporting the same sort of
thing. A late flow of dark reddish brown honey. We had thought at first
it was perhaps heather as it is the right time of year, but our latest
thoughts are that it is the balsam. If so it is the first time we have
ever seen it in this sort of quantity, ie. supers full of it. But
regarding the source of white pollen, that is a yearly occurrence at
this time, due to the balsam, and if you find the flowers you will see
it for yourself.
 
I would be interested to know if you have the same thing going on where
you are.
Where are you anyway - what area?
 
Regards,  Madeleine Pym