You need Article 11 of the
Air Navigation Order 2005.
The CAA and EASA have gone for the sensible option of making C of A's and Permits non expiring. However in order for the aircraft to fly it needs a valid Certificate of Mantenance Review (C of A) or Certificate of Validity (Permit). These certificates do expire.
This is not dissimilar to your PPL which, even if unexpired, is not valid for flight unless it contains a current Medical Certificate and current Class Rating. (All we need now is some joined-up thinking to make the PPL non-expiring as well)
Mike