The previous thread ...
Multilingual websites and internal link structure
... seems to have discussed quite a lot of the issues. But ...
I felt that the question of whether or not to host a French translation within a section of the existing (e.g. .co.uk/fr) site - or whether to host a French translation at a new .fr French domain was not agreed unanimously.
Therefore any extra thoughts on that aspect not already covered in the above thread would be appreciated, particularly in view of the fact that we are targeting French speakers IN THE UK more than we are targeting people actually in France. (So .. hosting any .fr domain actually in France may not be such a necessity, if we go the way of a separate .fr domain - or do you disagree?).
Other questions spring to mind:
Is building links for a new (.fr) domain harder (in terms of gaining in the SERPS) than building links for a sub-section of an existing site? The new domain Sandbox period combined with a bit of linkjuice from internal links would suggest so, to me... but is the long-term ranking potential better for the separate domain?