Stonehenge

The answer to that question really depends on how much you want to see Stonehenge. If it would fulfil a lifetime dream and this may be your only opportunity, then go for it. If not, then I would say spend your time in London, there's just so much there.
To visit Stonehenge would, I think, take the best part of a day (although I've never done it myself from London) especially if you're relying on public transport and not renting a car.

Hi Clint, I agree with Maggie. But if you really want to go and it is interesting to see... I have made this trip a couple of times by train. From London Blackfriars to Salisbury it takes about 2+15, same for the return journey. Without a travel card it costs about 65GBP total. From the train station or city center you will need transport to Stonehenge which is about a fifteen to twenty minute drive. It's a very short walk from the train station into town center and you get to visit the beautiful Salisbury Cathedral and Magna Carta for free. So - it will take most of a fun filled day.

The first time we went, we took a tour bus from London (which also took us to Salisbury and Bath). It took a full day but was great.

Clint, If Stonehenge is important to you, you may want to consider what we did 2 years ago. To get inside the circle and touch the stones, you can take a "Special Access Tour" that takes small groups (less than 40 when we were there) inside before and after the monument opens to general admission. It was not cheap, but we had a half day trip that included some other megalithic sites including Avebury. The vans picked-up from a hotel near Victoria Station & returned us there. The company is called Astral Travels. You can see what you think. Travel "must sees" are very individual. We had skipped going in to see the stones on 3 previous trips because we could see them from the road and would have been kept back by ropes. We loved getting to hang out with the stones "up close and personal".

If Stonehenge is something you've always wanted to see and you won't be this close ever again, you can try it. It is definitely doable as a day trip. We took the train from London to Salisbury, saw some of that, and then took a local bus out to Stonehenge. There are tours but I think you can do it on your own, unless they will save you time overall. I'll be in London soon for my seventh trip, and I still haven't seen it all. So there's always the next trip.