I would recommend www.institut-francais.org.uk which is the official French language institute based in South Kensington. They do all sorts of language courses, including business courses and tailored courses for corporates. They also have a lot of French cultural activities which can also be useful in getting teams up to speed with culture and attitudes (especially if you are going to regional cities and not just Paris).
I took a class there some years ago and it was a very good refresher ahead of a six-month stay that turned into three years.
A word of warning – if the work is in Paris, make sure you get a good expenses policy signed off. Same if its Nice/Cannes or Monte Carlo.
Let me know how it goes – slightly jealous at the thought of it!