Viator connect me with Subculture Tours in London. First rate all the way. I booked a private tour, as I had only a three hour window to see all the highlights. My guide, James, has a Masters in History, and swung me by eight palaces, the changing of the guard from the guard headquarters front row, the royal gardens, in the tube, through the alleyways, all the while thorougly entertaining me with dates and historical facts. This was my first visit to London, I had only four days and many business obligations. HIGHLY recommend!
We confirmed in advance with Viator and had booked this several months earlier. Our guide was a no show and we lost the morning and opportunity. We had a similar problem on the same trip in Gadansk
Poland. We had a great experience in a private car tour with Viator for the Leeds Castle and Canterbury Cathedral and White Cliffs of Dover we would give a 5 but very disappointed in these other two no shows. It must depend on the vendor.