replacing the glass in doors?

You will be required to use 6mm glass (it probably is already unless it's quite old). I presume you are thinking of K glass like the use in double glazing. Not sure how available that is and whether it would make a difference worth the price if not used as part of a double glazing set up. A curtain across the door(s) would be cheaper and a lot more effective, jeeping heat in and draughts out.

Nope! The requirement is for "Safety Glass". Could be 4mm Toughened or Laminated.

Low E glass coating on a single pane would have no heat-loss reduction properties at all. It must be inside a sealed unit for the reflective effect to work.

The only thing that will make a worthwhile difference is double glazed panels. Solid glass - no matter how thick - will conduct the 'cold'.

Go read the regs carefully.

In detail panes less than 250mm wide beween glazing beads (and with an area not exceeding 0.5m^2 but you never usually reach that) can be of 6mm annealed glass.

Above this size, safety glass is only required in windows where the glass is below 800mm from the floor and, in doors, where it is below 1500mm from the floor.

I believe "outbuildings" are not covered by these regulations - at least I was able to buy a ~1m square sheet of 6mm for my old garage. I should say this replaced a sheet of 3-4mm glass which my son had sent a football through. I'm not a particularly nervous or sensationalist type when it comes to safety but, had I been stood at my workbench at the time I would have been hospitalized given the huge area of the garage that was filled with medium size shards of razor sharp glass. The 6mm probably wouldn't have broken anyway and the weight of the glass would have limited the danger area - I feel a lot happier behind the 6mm!