Bristol is about 2 hours via train or car from London Heathrow. Thornbury is 15 miles from Bristol, and about 2 hours from Heathrow.
No, everything will be at the castle. The ceremony starts at 3pm and will run until about 11 pm or so.
Cocktail Attire- suits for men, and long or short dresses for the women.
You can hire a car (taxi) to transport you. If you are taking a taxi, you MUST reserve your ride home at the end of the night ahead of time (things will wrap up by around 11 or 12). Thornbury is a small little town, so no bustling cabbies all around. Call South Glos Taxis to reserve service: +44-011-797-98411 or visit their website here: http://www.southglostaxis.com/