It's a good problem to have, visiting London you might have so much to see and do that you just don't leave the capital - meaning unless you've booked another destination elsewhere in England as part of a larger tour, you're going to miss out on a very different part of life in Britain, everywhere outside of London haha
London is one my all-time favourite destinations, but it truly is like another country when compared to the towns and cities elsewhere across England. The pace of life alone is palpably different!
The towns and cities of England outside of London are like an entirely different country
But making the most of your time is important, too. You don't want to spend needless hours in traffic.
Bath is a great choice, it's only about 90-minutes by train if you're on you're determined to travel independently, but you can join one of countless bus tours which include at least one other world famous landmark with your trip to the beautiful city of Bath. Not only Stonehenge, but Windsor Castle!