Actually there is.
1995 British Waterways Act 17(c)
either—
(i)the Board are satisfied that a mooring or other place where the vessel can reasonably be kept and may lawfully be left will be available for the vessel, whether on an inland waterway or elsewhere; or
(ii)the applicant for the relevant consent satisfies the Board that the vessel to which the application relates will be used bona fide for navigation throughout the period for which the consent is valid without remaining continuously in any one place for more than 14 days or such longer period as is reasonable in the circumstances.
Yes there is an exemption in (ii) for boats being "used bona fide for navigation throughout the period for which the consent is valid", but boats CMing do not pass this test.