Can you exchange old French Francs notes for Euros in the UK i.e. Is
there a branch of the Banque de France in London? and what exchange
rate can I expect?

I did not find a Banque de France branch in London, but there is
something ese that should be useful.

Although aimed at Americans, this advice by the French embassy in the
USA should work for British citizens, too:

"Any customer with old French banknotes may: 
- send his French banknotes before 17/02/2012 for current series of
banknotes by registered mail to : Banque de France, Caisse Générale,
Service 18, 10 Boulevard Duclaux, 63407 Chamalières Cedex - France to
exchange against euro baknotes if the amount is less than up to
5000EUR.
The amount will be rounded at the lower 5EUR unit and a postal fee may
be charged according to the amount and to the destination country.
Coins are not allowed."
http://www.ambafrance-us.org/atoz/euro.asp#8

Scriptor