I went to Spa back in 2008 and 2009.
Camping is plentiful, but I advise going actually booking a campsite, such as that of the Elephant (Contacts can be found on this site, Under spectators experiences)
In 08, we just arrived and saw what our options were, while there a plenty, toilets are a row of portaloo’s, never cleaned or emptied and no showering facilities.
In 09, we booked the Elephant, which offered us actual Toilets and Showers. Reasonable price and a good bunch of F1 Fanatic readers stay here!
Both years we’ve spread the driving over 2 days. We drove to Dover, stayed the night (as were from on the coast of West Wales even thats quite the distance!) And then caught the early ferry Thurday morning, drove to the track, and went for the pitwalk, on the way back, we normally stay in Dover, and then catch the early ferries again. (My dad booked these, so, knowing him, they were likely much cheaper in the mornings)
The camping was fine on both years, although (Minus the last 10 minutes in Spa 08)it remained dry all weekend for us.
Feel free to ask any questions about the facilities in the track, walking to the track or food etc. Theres a lot I could say so specific areas of confusion would be nice.
And for all I know, Maybe I could be seeing you there next year! (Although im off to Uni then, busy times!)