I am considering taking the Best of London 7-day tour. Two things that I am interested in that are NOT included on the tour are side trips to Greenwich and Stonehenge. Is there enough free time on the free afternoons of this tour to fit in one or both of these sites?
Greenwich yes Stonehenge no.
I agree, no especially for Stonehenge, however why don't you extend your stay after the tour to be sure and get to both and really enjoy them?
Stonehenge is not impossible, but you won't get to see anything else of Salisbury (including the Cathedral) which seems a huge shame.
And you will only see Stonehenge itself, not get to see the wider landscape context in which it fits.
It is 90 minutes each way from London Waterloo to Salisbury every 30 minutes- so if you caught the 1320 train (preferably the 1250 train), that has you on the last bus up to Stonehenge with just over 2 hours there before the last bus back to Salisbury station.
Booked ahead of time you should get the return train trip for around £20 with Advance train specific tickets.
So there is a technical answer of yes you can get to Stonehenge, but a realistic answer of, is that what you really, really want to do with that free afternoon?
I recommend adding a couple days to the end of your tour. The Premier Inn County Hall would be a great place to stay. Waterloo station is a short walk for the train to Salisbury and you’ll be on the South Bank to take a boat to Greenwich. I’m also going to recommend visiting Salisbury Cathedral.
I’ll be on my second RS tour this year and add time to the beginning and end to make the most of my trip. I’ve travelled solo to London several times and always find a reason to stay longer and keep returning.
Happy travels!