1. For efficient heating of the space overall I should stick with
radiators since these are running on gas anyway, therefore I'm
producing less carbon if I run uninsulated on rads rather than pretty
well insulated on the electric UFH.
---This is OK and what I plan to do anyway

Indeed.

2. If I stick with putting in the UFH for comfort heating then I am
wasting my money with the 6mm insulation, since most heat comes from
the rads so payback on the insulation is somewhere well over the
horizon, probably beyond the lifetime of the installation.
---This is OK since I don't have the money or the headroom.

What is the intended finished floor surface? If carpet or wood based, I
wouldn't use UFH at all.

3. For control of the UFH I should use the floor temperature sensor and
set it as low as possible (say 20C) to keep the duy cycle to minimum so
it just does the job of keeping the floor at a comfortable temperature.

Certainly. Personally, if I really wanted UFH heating in there and there
really was only a choice between 6mm and 0mm insulation, I would still go
for the 6mm. Note that 10mm boards are the same price and the additional 4mm
is worthwhile.

don't seal the joint then any condensation which forms on the inside of
the wall membrane should (on the whole) run down the gap between the
wall and floor membrane and so end up in the drainage system rather
than on the floor or at the base of dry-lining.

What you haven't considered is that if the insulation is adequete, then the
wall temperature will be much higher. Provided the ventilation is up to
spec, which seems to be very much the case with heat recovery ventilation
mentioned, then the relative humidity at the vapour barrier will be much
less than 100% and no condensation at all will occur. As the temperature
drops through the insulation, what little air (with closed cell insulation)
that gets through will be very low in absolute humidity due to the vapour
barrier, so no interstitial condensation will occur there either.

Christian.