I’ve looked at a couple of websites that seem to offer garage reviews but the local garages don’t have any reviews (or have only one giving it 5 stars which I don’t trust).
Is there some other way of finding a reputable garage (it’s a Volvo with low brake fluid warning) in West London?

https://www.goodgaragescheme.com/

Maybe?

Look on owners forums and try to pick a specialist who often/only deals with your make of car. For example I drive 20 miles to my garage but they only deal with one brand of car, can do things like swap out parts from their stock and only charge you for what fixed an issue, rather than for each part until a problem is solved.

They charge me less than kwik fit and only marginally more than DIY.

Have you checked the brake fluid level yourself?

There’s a good one in Berwick.

This website/forum is a good resource, try joining and asking:

http://www.volvoforums.org.uk