The email I have from HeliAir doesn't specifically mention C. It does mention A & B, and that as a result of B they must be satisfied you'll be safe. Seems fair enough to me.
The details of the practice days haven't been released yet; but I would be disappointed if there was anything other than a minimal charge for them given what they are charging for the slots themselves and ancillary bits and pieces; plus the fact the practice day is mandatory. (I suspect they will make money from the practice day if their own SFH customers are also flying into Silverstone!).
HeliAir have always been a fair bunch to me, so I can only speak as I find - if you want clarification give them a call!!
C.