Running 2wd one day and 4wd the other will not be a problem as long as you book in the class of your choice for each day.
Ive just been informed by a committee member of the Hinckley club that its a set of tyres per meet (DAY), so if you are paying the 18.00 pound for the two days you will get 2 sets of tyres and if your paying 10.00 pound for one day you will get one set of tyres.
Now thats a deal that cant be missed