I am planning to visit Glastonbury for a few days and on my return, to stop over one night in Avebury, then visit Stonehenge (or this visit can be done before getting to Avebury) and then catch my flight at Gatwick airport. What would be the cheapest option to travel from Glastonbury to Avebury/Stonehenge? How can I then get back to Gatwick airport? Thanks.