Of course beaches will be busy, it's hot weather and everywhere else people would go on a hot day is closed to the public. A lot of people, especially in London, do not have gardens to relax in.
As for social distancing, I simply cannot believe that people will be sitting or lying less than six feet from a stranger. Six feet is not very far, really. The real issue will be groups of people meeting up together, but that is inevitable.
People don't sit within 2m of another group on beaches generally. That's way, way too packed for it to be enjoyable. Typically it would be far more than 2m.
This whole beach furore is threefold in my book:
1. Justifiable complaints about obstructive parking and littering, both of which are already an offence and just need enforcement
2. Complaints about people parking in residential areas, to which the answer is "tough, petition your Council for a Permit Scheme if it bothers you that much". (See also: streets near railway stations - though not always - there is plenty of non-Permit street parking near Bletchley station if you wanted it but people mostly behave!)
3. NIMBYism from people who have quite enjoyed having their beautiful tourist hotspot without the tourists. Well, er, tough, if you don't like tourists don't live in a touristy area - literally all of them have been touristy areas for years, potentially well over 100 years, so definitely within the timescale that any current resident purchased or rented their house. Odds on if you do the tourists are making a considerable contribution to your wage!