exactly as above, your 5 x 3m isn't exceptionally big workshop a 2kw fan heater will more than suffice, and if you get one that runs on thermostat you could keep the workshop heated to say 10-15c at lowest setting. I have a similar workshop and i only use heating when I need it when I am working from it, and my 2kw heater heats it up well within half an hour on 2kw, and I lower it to 1kw just to maintain 20c , mine is a cavity wall construction with flat roof.
My workshop is detached from the main house by 60 feet and I used pvc drain pipe to carry 2.5mm T&E and 1.5mm T&E for lighting through this water tight drain pipe, I have had no problems so far despite 20 years now, and I have a separate RCD for this and it has never tripped, the pvc pipe is running between 1.5 to 2 feet underground.
My workshop is detached from the main house by 60 feet and I used pvc drain pipe to carry 2.5mm T&E and 1.5mm T&E for lighting through this water tight drain pipe, I have had no problems so far despite 20 years now, and I have a separate RCD for this and it has never tripped, the pvc pipe is running between 1.5 to 2 feet underground.