>>> Is it a public holiday in England today ? If not, here do all these people come from ?
All further and higher education establishments (i.e. for post-16 and post-18 education) are currently closed, except for some limted face-to-face contact time for some 16-17yo students who are undergoing further education within a school environment. (I note that many of the people who're shown in photos of crowded beaches appear to be around 'student age').
All secondary schools are closed except for those students who're mentioned above and for 14-15yo students (who might only be going in on certain days of the week).
All primary schools are closed except for some 5-6yo and 11-12yo pupils, who might only be going into school on some days of the week.
There are also areas where local councils have advised ALL schools (both primary and secondary) to ignore the national partial lifting of lockdown restrictions and to remain totally closed anyway.
As at June 14, there were 9.1 million workers 'furloughed' from their jobs, with the government paying 80% of their wages. THe re-opening of shops, etc, will have reduced that number but there are plenty of others, such as those who work in the hospitality industry, who still can't return to work.
There are also lots of self-employed people (such as hairdressers) who can't yet re-open their businesses.
So there are LOTS of people in the UK who, having been bored stiff sitting at home for the past three months, are now tempted to take advantage of the sunshine and head for the coast.