I don't want to state the obvious but I may have misunderstood your question. You can watch racing for free in your camping chair. Find a spot around the circuit (that isn't prohibited) and enjoy. You can buy event tickets for grandstand and fan zones but I would thing the grandstand is sold out by now. Good viewing spots do get busy in race week so you'll have to arrive early - some (on private land) you have to pay a few pounds for.

Camping is likely to be pretty difficult to get at short notice. I've already booked and paid for mine and I know our camp site is full. Try phoning the campsites and see how they operate their site. Failing that you might be able to get a homestay but without knowing dates it might prove tricky.