Edit Update: Officially announced it was a couple of Typhoon aircraft responding to an emergency that broke the sound barrier.
http://www.bbc.co.uk/news/uk-england-17697328
My thoughts are it's possibly something geological, though we aren't near any plate boundaries so the chances of that are very remote. Sonic boom has been ruled out by the airport.
It also covered a huge radius (I personally heard it in Swindon and it shook my house's windows and my chair).
Any ideas/thoughts on what it could be?