The Speedfest is held on the Indy circuit rather than the full GP. You will be able to access the inside of the Druids Hairpin for panning shots around the top of the corner, and as they go down the hill, and if there are trees or shrubs you can pan through the trees. The outside now has quite a lot of fences, but from the top of Druids there is a photography window - but it may be occupied, otherwise you need to get the lens as close to the fence as you can.
At Clearways at the other end of the circuit, there is another photography window, but can't always use it.
You should be able to get access to the inner paddock by either the pedestrian tunnel or the car tunnel, but you probably shouldn't get access to the pit lane itself.