Thanks for all the helpful comments. On the subject of comfort, all the panels have really thick insulation (the floor insulation is 100mm thick) and it has domestic grade double glazing. The electrician will be fitting a 1.5kW thermostatically controlled heater, which the fitters tell me I will only need on for a short while before the room becomes warm. All the windows are opening so, in summer, it should be reasonably OK for temperature.
On the subject of security, all the windows have key-operated locks and the door is a standard domestic grade multi-point locking affair. In fact, the security is probably better than on the house, apart from the house alarm. The train shed is situated at the bottom of the back garden which can only be accessed by coming through the front gates and past the side of the house or through a hedge from one of my neighbours gardens. The front gates are over 2m high and the neighbours all have gates as well. To get through from my neighbours gardens an intruder would have to breech thick hedges. Foxes and rabbits manage to get under the hedges and squirrels over them, but it is not a very inviting prospect for an intruder. I will probably fit an alarm anyway.
I have re-measured up today and there's a slight hitch. I've decided that the 2 ft gap across the doorway is insufficient and so have had a slight re-design. I held back on ordering the baseboards for this very reason. The revised layout plan is shown here.
I have had to replace each 4 ft X4 ft board for the storage loops by a 4ft X 2 ft board plus a 4 ft X 18 ins board. Also I have had to add to triangular corner boards to accommodate the branch run rounds. There will be 150mm less for each route to and from the loops, but the up to down (and vice versa) run will still be about 14m (well over a mile in scale distance). Otherwise the layout remains unchanged. I will order the baseboards today. I'll post some more pics when the electrics are installed.