Leigh Delamare
Oh! I stop there often on my jaunts from West Wales to Gatwick airport, so I know that it wasn't there that they filmed it. Also, when Gavin and Smithy are on the motorway they pass signs for Chepstow and Magor on the Welsh border. Also Leigh Delamare has a westbound and eastbound side so the sign showing M4 Cardiff and M4 London wouldn't be there as you can only go one direction from each stop. I know, I sound like those sad anoraks that come on this website and point out the most ridiculously obscure goofs that only they know and the rest of us couldn't give a sugartits about. Cracking.
Life isn't a rehearsal, so make this one your best performance