Welcome to the joys of the intricate rail network around Farnborough I have 5 stations within 1.5 miles of my house, on 4 different lines!
Despite living here Im not an expert but my understanding, is that from Camberley you can buy a ticket to London routed via Ascot or Ash Vale. The route Ascot is cheaper but can only be used that way. The route Ash Vale is obviously valid via Ash Vale and Brookwood, but not Farnborough, Im not sure whether you can also travel via Ascot on the basis that it is more expensive but I dont think so?
Farnborough tickets are valid via the main line, via Brookwood and also via Redhill or Reading, although the later costs more.
So if you wanted to travel to London from Camberley and back to Farnborough, via Ash Vale, you would need a Brookwood to Farnborough single. Going via Ascot, I imagine you would need an excess of some sort to come back to Farnborough.
Hope thats some help and that someone with more ticketing knowledge than myself may be able to fill in the gaps