Honestly, I believe the French do this much better than we do over here (it's sadly not seen as a career to aspire to in the UK, where it seems to have more respect in France). If you've received such training / experience in France, I suspect you would already have strong skills that would be very useful.
- looking for 'temping' agencies who may be useful at matching your experience to restaurants seeking short-term staff to cover for absences
- Contacting suitable restaurants/hotels direct. The greater your prior experience, the more useful this option should become
In terms of areas, fine dining in London is spread (geographically) very wide.