It depends how high you mind the planes being - not many places completely avoid them these days.
We've been to Dungeness today, that's pretty isolated but in easy reach of civilisation if you should need it - amazing place, but it does have a nuclear power station on its doorstep :-)
Big chunks of Hereford would meet your criteria. Takes ages to get anywhere as there are hardly any major roads, no planes (just helicopters around here), very quiet and hardly any big towns or cities, must be loads of little places to choose from if you have a look.
You are describing lovely Dorchester in Dorset. It is the county town of Dorset but is a very small town, where Thomas Hardy lived and wrote. The nearest big-ish town is Bournemouth. Bristol is further out, and the nearest airport is Bristol and Southampton. No motorways. If Dorchester is still too town-ish, there are many lovely villages surrounding it in countryside that is second to none. Type DT2, DT3 into Rightmove.co.uk for villages and DT1 for Dorchester.
The Farne Islands, off the Northumbrian coast. It meets all four of your criteria, provided you are happy with the screeching and bucketfuls of birdshit.