Welcome to the forum. I haven't been to any myself, but if you google keto restaurants in London, there seem to be quite a few. Hope you enjoy your visit.
Very few actual keto restaurants- they are too costly to make a profit- but most ordinary restaurants will make adaptations for you, eg serving salad or veg instead of fries.
The worst are tea shops or coffee shops were all food is carbs
I've not been myself but, https://www.caldesi.com/caldesi-in-marylebone, the Chef/owner was/is a Type 2 who has worked in collaboration with Dr Unwin and written a couple of low carb/keto cookbooks which are very good