Not sure if it's a London thing. In my city neither I, nor the people I know rely on Google maps reviews for picking restaurants. We either know the place, follow a recommendation or try the place based on menu, price, looks, vibe, position etc.

A week ago I went to Venice and I only looked on Google maps to see what the menus and prices are, but I wasn't interested in the reviews themselves or the grade, bacause IMO, people have biases. One evening we went to one of the restaurants I spotted on Google maps but the rest of the evenings we wandered the streets, and picked what was close, if we liked the menus, the prices and the atmosphere.

One of the restaurants had only 3.4 grade on Google maps, few reviews and mostly locals ate there. The food was very good and the service was great.

I do not generally make my mind based on reviews from Google Maps, Booking, Amazon. Of course, if the overall grade is very low, I will give it some thoughts and maybe read some reviews. But generally I don't make a decision based on reviews.