Just to update on the batteries
My potential installers seem to think its not an issue, they say they install approx 6 systems a week and have no issues (raised) in regards systems not charging. The cover most of east anglia.
So maybe we just haven't had long enough sub zero cold spells to trigger that, or the BMS is useless and allows charging and permanent damage to occur.
Of course also the chance that some customers would have issues but no idea, missed charge days, or degraded performance.
The more expert (in regards batteries) one of the team is out quoting today so they said I should get a response later on today.
I am inclined to take the risk.
If there was a very cold snap, or prolonged period you could always override them and not charge them at all.
Also its possible to go below zero degrees but you have to increase the charge time, above zero for Lithium its 0.2 max, so 20% of the capacity incoming, as the temp drops its down to 0.1 below zero and 0.05 as you approach zero F.
Although so many lithium based techs its hard to know if this specifically applies to the chemistry they use in home batteries.
Oh and FWIW, was reading up about insulating and seems thats actually not that beneficial.
All you can in effect gain is an increase in the time that it takes for them to drop in temp, so a prolonged cold spell of very low temps will mean no insulation would make any diff.
I would probably need to make them a simple cabinet anyway to avoid direct sunlight, which may trap a little heat, and for the minimal cost I would probably semi insulate it, I mean it can't do any harm right.
I can get metal knocked up by the fabricators at work, so maybe a simple metal cabinet with 25mm PIR and a few vents to vent out near the top may be good enough to give some benefit it most scenarios.
First job would probably be to actually measure how cold it can get based on listed outside temps. Its quite sheltered where they would be, so quite feasible its a bit warmer than the headline temps anyway.