Long associated with mystic celebrations, ancient monuments and the Green Man (whose face crops up all over the place in medieval architecture), the Summer Solstice is celebrated across the UK. It's a great excuse to visit some of Britain's most magical and mysterious places and, well, have a party near them.
Steeped in the legends of King Arthur and other assorted mysteries, Glastonbury is definitely a top spot to celebrate the Solstice. The top of Glastonbury Tor is the preferred place to see in the sunrise on the longest day of the year; get a spot right at the top next to St Michael's Tower for spectacular views across the area. It's a slightly calmer affair than the Solstice party at Stonehenge, so you can relax and take in the sights with an air of tranquility.
Probably Britain's most famous ancient monument (thanks to the mysteries surrounding its creation), Stonehenge is also the most popular spot to celebrate the Solstice. On this day, visitors are allowed to get much closer to the stones than usual, and usually a fairly wild party ensues. For the rest of the year, it's an extremely tranquil spot, and the views over Salisbury Plain are gorgeous.
The megalithic stone circle at Avebury is the largest in the world, so Summer Solstice is a pretty big deal for the village. You can walk in amongst the stones at all times of year; explore the village of Avebury itself; and then settle in the nearby pub for a drink or two, just as the ancient Britons did (probably).