G&L , like SPGS feeds from central London preps which on the whole are non-diverse- wealthy white families from SW London , W London, the Nappy Valley across the river. Its odd, as its a top school, but doesn't get that London wide attendance like say CLGS , which has a much more diverse school population. I guess because its still a local school rather than a central London one.
You have to balance this - do you mind your DD being very much in a minority but at an excellent academic school- I'm positive the school and the girls there will ensure she fits in , but I understand your concerns.
We chose Old Palace as we wanted our non-white DD to see lots of girls like her, but despite sending 70% of its leavers to Russell Group Uni's its not stellar academic like G&L.
City of London Girls as I mentioned is more diverse due to its ease of location from all directions of London, and JAGS has more diversity as its placed in South London, albeit a very white affluent part of it.
I'm sure your daughter will thrive at G&L, our decision was easy as our DD was at OP since pre-school.
In your shoes, I'd send her and see how she got on, I'm sure she'll see a few girls that look like her, which should be enough for her not too feel completely alienated, and I'm sure she'll make lots of friends who look nothing like her.