The closest train station to the festival is
Castle Cary although
there aren't many discount fares available but if you book early you could travel from London Paddington for
£56 return per person if you
plan on returning on Monday 29th June. If you plan on travelling back
from Glastonbury on Tuesday 30th June then fares start from £38.50 return per person.
There are discount fares available to Bristol Temple Meads with trains operating from Paddington every 15 or 30 minutes. You can travel from London Paddington to Bristol Temple Meads from just
£38 return if you are returning on Monday 29th June. When you arrive in Bristol you can then get a coach to Glastonbury from
£12 each way.
So it seems that if you're going to the Glastonbury Festival from London, it's worth checking out trains to Bristol and getting the coach which could save you time & money but remember to book before the day of travel otherwise you could be in for a shock. Fares on the day of departure could be in excess of £100 return.