When I met them - end of o7 I believe - they had indeed a very small office in London but it had not expanded significantly since its opening 2-3 years before.
Even in France they are quite small.
When Investance was created it was quite ambitious but after a few strategy projects they refocused on very operational topics. Though they boast being "industry experts" they work mainly on IT-driven projects as 90% of French consultancies. If you fancy writing functional specifications for a private bank that deploys a software they have the job of your dreams. If you are interested in management consulting, go your way.
Maybe it's better in London, maybe...