Single space. I always single spaced, but then changed to double space after I did a lot of work for a certain UK engineering company whose style guide is militantly double space to the point where documents will be rejected for not using double space, even if they are otherwise perfectly correct. Same company also has an odd demand that a space is placed between numbers and units. E.g. 10 kN and not 10kN. To my eye that extra space looks horrible.
Double space was made redundant years ago by modern word processors. I have since gone back to single space, because it is the correct way, but will do the double if I have to just please some jobsworth. Just goes to show that the UK engineering industry is still populated by dinosaurs. It is pointless rejections of deliverables for reasons like this that create unnecessary delays and cost overruns. The big players in the safety-critical industries are all guilty of it.