I'm in London, and I'm not familiar with the camp sites around Inverness - there are quite a few though, more than around Falkirk I would guess! Here's one which is just to the west of the town and only 10-15 minutes drive from the stadium - or a long walk via the local pubs... :022:
http://www.bunchrew-caravanpark.co.uk/gallery.aspx
For the journey from Folkestone to Inverness I would recommend the route via M20, M25 anti-clockwise round London, M11, A14, A1, A66, M6, M74, M73, M80, A9 - the traffic on the M1 and M6 southern end can be horrible, and this route's usually quicker. I've done the journey from the start of the M11 to Inverness in 7 hours 20 minutes before (yes, I know that's an average greater than the speed limit!!), but realistically you'll probably be looking at around 10-11 hours driving from the tunnel. Good luck and enjoy the journey and match!