Preparing to bow to someone with better knowledge, Shy I think you are quoting an old rule that no longer exists.
The current Rule number you refer to is nothing to do with landing in congested areas:
5.—(1) Subject to paragraph (2), an aircraft shall comply with the low flying prohibitions in
paragraph (3) unless exempted by rule 6.
(2) If an aircraft is flying in circumstances such that more than one of the low flying prohibitions
apply, it shall fly at the greatest height required by any of the applicable prohibitions.
I cannot find, in the whole of CAP 393 as issued March 07, any reference to needing permission from the CAA to land in a congested area. The only rule that affects is for Special VFR flights:
(c) Special VFR flight and notified routes
(i) Subject to paragraph (ii), any aircraft shall be exempt from the 1,000 feet rule if—
(aa) it is flying on a special VFR flight; or
(bb) it is operating in accordance with the procedures notified for the route being
flown.
(ii) Unless the written permission of the CAA has been obtained, landings may only be
made by an aircraft flying under this exemption at a licensed or Government
aerodrome.
So if flying SVFR or on a notified route you must land at a licensed airfield unless you have the CAA exemption. Edgbaston : SVFR ??