Lots of London pubs serve quite acceptable, traditional English food, which although good and well prepared might pass as "bland" for this purpose. For example, a pub that specialises in pies might be just the thing.

Lots of pasta dishes in Italian restaurants are quite bland, in a way, and it's a reasonable Italian restaurant will be able to "mix and match".

I think, too, that given that your friend is pregnant, most reasonable restaurants will understand the point of your request (if you make it) and try to be as helpful as they can.

Whereabouts in London would you want to eat?