Scratch Paris, I Meant London Kosher restaurants
There are loads on Golders Green Rd, as well as on Finchley Road in an area called Temple Fortune. Other areas have some as well, notably Hendon and Edgware, but GG is really the core of Kosher cuisine.
There is a terrific falafel place on Golders Green Rd run by Israelis called Pita Plus or somethng, and there is a high-end expensive place called Bevis Marks in the city (financial district -- center of town) which is pretty good as well.
Good luck !
There are a few sites with listings (not all of them up to date):
I'll confess to not having tried any of these places! Most of them seem to be concentrated in the northern residential areas, not so many are centrally located.
The range of choices might depend on how strictly the rules are interpreted. For mildly religious people there are lots of places that meet the (similar but not identical) Islamic dietary standards that ought to be fine, but this would probably not suit people who hold to stricter interpretations.