There is no reason (legally) why not.
But notice that all the successful beach landing sites are in comparatively sheltered bays with very little slope. The reason for this is the sand becomes hard packed and not disturbed by wave action. Soft sand is a death trap unless you have tundra tyres (pretty rare in the UK!).
My local beaches are possibilities - but only at the bottom of the tide where the 'flat' 'hard' bit is exposed. But that is only for a couple of hours maximum and there is no easy way to get further up the beach.
Talk to some amphib pilots about how they hate trying to get to the upper part of beaches (very small wheels make it near impossible!).
In BC the 'accepted' technique is to carry out a series of passes running the wheels along the beach and checking for the amount of drag and the depth of track - its something that needs practice!!