Our route from East Sussex has always taken us over to Southampton and then up past Salisbury and a bloody beautiful country route over to Pilton from the east. I think the last time we went via the 303 or any motorway was in 95!
There's plenty of other ways over to Pilton that do not involve having to hit the motorway, its just a question as to how much time you want to take. We all know there are some bloody lovely laces to see and visit en eoute, whatever route you take.
Consider solstice at Avebury...its on the Sunday this year. A group of us went this year and also took in Bristol for the Banksy show. Made for a lovely extralong break. No longer welcome at Old Oaks campsite in G Town though!
Stay in Bristol the night before and then go cross country to Pilton instead of the M5?
Pre-plan your route to take in some of the many cider houses local to the area? Heck's in Street sells the best cider on the planet - Bramley Apple & Honey - as well as the Single Variety Perry.
Head to anywhere within 50 miles of Pilton that has a B&B with room still spare (eg, Bridgewater, Taunton)...I highly doubt there is anything free in Glasto Town/Shepton etc cos of what happened this year.
B&B/hotel/campsite...just get a map and choose!
Travellodge have super cheap deals most of the time...again just pick one nearish.
Make a wee extended holiday taking in another part of the west country the weekend before.
Decisions decisions decisions...
whatever you choose, book early is probably the best advice. We all know what happened this year and noone wants to go through that experience...kerching to all the tourist accommodation locally next year!!!