In addition to the services listed below there are also frequent coaches from many towns & cities (and Heathrow Airport) to Bristol, where you can connect with the 376 local bus to Glastonbury.
Connecting Glastonbury, Street & Shepton Mallet to London with a single service. Coaches depart London at 17.45 and Glastonbury at 08.30 (Sundays & Bank holidays have different timetable). Timetables & Fares on the website. Select Superfast 3 from the options.
National Express Coaches
There is one daily National Express coach service (403) that directly connects London and Glastonbury. That departs London at 18.00 and Glastonbuy at 06.50 – hence it is not possible to do a day trip from London by coach. However you can travel to Bristol and then get a local 376 bus to Glastonbury.
Low cost inter-city travel with several services to Bristol from where you can get the 376 local bus to Glastonbury.