So basically they didn't have a contingency
I appreciate that they don't own the fields used for temp parking. Ok fine.
Locate and secure hard surfaces within 15 miles (or further if need be) of the circuit, and run a shuttle bus service. Negotiate with the field owners to use rubber matting.
Milton Keynes is just 17 miles away. Secure the parking used for MK dons stadium.
Northampton is even closer. Hire the University car parks, the Rugby ground car parks etc..
Negotiate with transport companies and have buses brought in from across the region, or country.
All of that would probably cost less than the loss they will make from up to 30,000 spectators missing out, and the terrible PR they are receiving again.
Which they do, for Saturday and Sunday. The difference is that the number of buses they need simply isn't available to them on a Friday.