Your guess is incorrect. Whatever the previously expressed intention of the CAA, the extent of the permission granted by the ORS is clear.
SERA.5005(f)(2) reads:
elsewhere than as specified in (1), at a height less than 150m (500 ft) above the ground or water, or 150m (500 ft) above the highest obstacle within a radius of 150m (500 ft) from the aircraft.
The ORS specifies that the permission in relation to this requirement applies only to flight within 150m (500 ft) above the highest obstacle within a radius of 150m (500 ft) from the aircraft but does not include permission to fly below 500 ft above the ground or water. This has the effect, in the UK, of amending the SERA requirement to read:
elsewhere than as specified in (1), at a height less than 150m (500 ft) above the ground or water, or closer than 150m (500 ft) to any person, vessel, vehicle or structure except with the permission of the CAA.
It may well be that this is yet another cock-up by an Authority that (as its middle management now openly admits) is in terminal decline but, until it is rectified, VFR flight below 500 ft above the ground or water is not permitted unless taking-off or landing or with the permission of the competent authority.
Should anyone object to flight along a 'deserted' beach at 20ft, one would be hard pushed to find legal justification before the beak.