Dragpin
The Permission the CAA refer to is in relation to rule 5 2(c), its a permission not an exemption becasue the rule says
(c) The 1,000 feet rule
Except with the written permission of the CAA, an aircraft flying over a congested area
of a city town or settlement shall not fly below a height of 1,000 feet above the highest
fixed obstacle within a horizontal radius of 600 metres of the aircraft.
You don't get an exemption from them, you get a permission from them, its a terminology thing, still amouts to fill in a form pay the money and get a piece of grey paper that says what they expect you do in addition to not endanger anyone while you land there.
If you do apply for one ask for an annual permission, and make it as unrestrictive as you can on yourself, you can get away with not necessarily specifiying registrations and pilots if you speak to them and annotate the form that way.