Have they really? I've see that old chestnut in many articles written about the IMC rating, but you tell me where the CAA makes such a statement.
This is the closest I've seen:
Safety Sense Leaflet 23: Pilots: It's Your Decision
More than three quarters of the
pilots killed when they lost control in
IMC were flying in instrument
conditions without an instrument
qualification. Disorientation can affect
anyone, particularly those who have
not been adequately trained to fly on
instruments, and kept in practice. It
is important to be able to see and
recognise cloud ahead early enough
to avoid it safely. Even an IMC rating
does not impart sufficient skill for
prolonged, intentional flight in
instrument conditions. Unless you are
in regular instrument flying practice it
should only be regarded as a
minimum skill to ‘get out of trouble’
if an unintentional excursion into IMC
occurs.
"Unless you are in regular instrument flying practice" is, of course, an important conditional.