It's bad practice because you are breaching the cavity as well as adverse effects of insulation on the cables. Also how do you draw the cables in without damiging them. I'm doing inspections for a local authority and if I find cables in the cavity the house will be rewired.
NICEIC TECHNICAL MANUAL
NHBC Requirements
It is noteworthy that the National House-Building Council states, in Section 8.1 - S2 of NHBC Standards, that no cables other than electricity meter tails are to be located in the cavity of an external wall. Where meter tails do have to pass through the cavity, however, this does not preclude the requirements of BS 7671 referred to in this topic having to be met.
APPROVED DOCUMENT C
Cavity external walls
5.12 Any external cavity wall will meet the
requirement if the outer leaf is separated from
the inner leaf by a drained air space, or in any
other way which will prevent precipitation from
being carried to the inner leaf.
Technical solution
5.13 The construction of a cavity external wall
could include:
a) outer leaf masonry (bricks, blocks, stone
or manufactured stone); and
b) cavity at least 50mm wide. The cavity is
to be bridged only by wall ties, cavity trays
provided to prevent moisture being carried to
the inner leaf (see paragraph 5.15 for cavity
insulation), and cavity barriers, firestops and
cavity closures, where appropriate; and
c) inner leaf masonry or frame with lining.