Do people have reviews of Tours By Locals? Viator has come recommended but some of their tours are already sold out.
Viator are a 3rd party re-seller, they don't run any tours.
I have booked through Viator in many countries and have always been assigned excellent guides. They are a tour consolidator and will actually hook you up with a local tour. I have never hired a local guide on my own - but there are several listed in the Rick Steves Travel Guides.
Came home last night from London. Highly recommend "London Walks" there.