Casual bookings

We will be visiting from South Africa in May and June and will be touring from Surrey up the east coast to Scotland, then down the west coast, more or less, as far as Plymouth then back to Surrey.

We can't predict where we will be at any time because we might like a place and stop there for a few days and, in any case want to be free to wander about without any commitment through pre-bookings. Is it going to be workable for us to phone a site say at midday and make a booking for that night, or are they all likely to be full with organised people who have pre-booked weeks ago?