Some of the ones we use which are rarely/never full, or don't take bookings. Am a bit reluctant cos someone else is bound to come along and say they hate the site, but we like these for being generally low key relaxed and (importantly) not need booking, or else we phone a day or two before to check if they're likely to be full:
Lakes: Wasdale Head NT site. One of my total favourites, no cars on site, surrounded by fordable paddlable streams and trees. We have been there in hot sunny weather and it was idyllic.
also Lakes: Side farm on Ullswater is quiet, doesn't get booked up massively, is right on the lake.
Peaks. um, ok I've got them in my head but forgotten the names! sorry
North Wales. One on top of a hill at Llanbedrog in the Llyn peninsular. nice walk down to a fantastic beach and little arts complex. again, I don't know the name cos we just turn up.
Also Penrhyn farm in between Colwyn Bay and Llandudno, just a farm site, looks plain and unexciting, but you can walk or cycle to the beach or to the town or go up to the welsh mountain zoo.
Scotland. So many up the west coast and we never book ahead but Achmelvich north of Lochiver is very good, and there's a tiny site by a garage by the beach in Poolewe, and Red Squirrel in Glencoe.
Cornwall. again I forget the names of these little sites but we camped at one just east of Tintagel in August without booking, you could walk down to a lovely little beach. Bossiney farm or similar.
Devon. I know loads of sites in devon but am guarding them jealously. But North Morte farm near Woolacombe is very nice, doesn't take bookings, you can walk down to a quiet beach. Similarly Mount Folly Farm in Bigbury. Upper Rew campsite outside Salcombe is also not fully booked even in August. Dartmoor has lots of small nice-looking sites but we ten to camp rough there.