Andy Hall wrote:

I agree that one would have to try pretty hard to breach tghe armour
and insulation, but it does say that all cables have to be insulated
to the standard for the highest voltage

There's more to it than that. BS 7671 Reg. 528-01-02 allows six ways in
which Band I and Band II circuits can be mixed in the same wiring
system. To précis, very briefly, these a

(i) all cables insulated for the highest voltage present;

(ii) cores of multicore Band I voltage cable insulated separately or
_ collectively for the highest voltage present;

(iii) cores of Band I circuit separated from cores of Band II circuit by
* an earthed metal screen with current carrying capacity equal to
* that of the largest Band II core;

(iv) compartmented trunking system;

(v) physical separation barrier when cables are on cable tray or
* ladder;

(vi) separate conduit, trunking or duct.


For method (iii) the condition is met by the armour of 2-core SWA cables
(to BS 5467) up to an including 10 mm^2. Since the OP mentioned 6 mm^2
SWA the arrangement originally proposed complies with BS 7671.

But I agree that direct burial of the SWA is preferable. It increases
the current rating too.

--
Andy