I would suggest getting them going sooner rather than later, they may well be OK under glass pretty much straight away, although they might grow a little quicker in the warmth of a window cil. As you are probably aware it's a compromise between light and heat. I tend to feel that I get larger plants, but more slowly developing, in the greenhouse. I believe that Western Scotland tends to get more sunshine earlier in the summer, so it seems to make sense to try to get things going early on.
John Innes #1 compost will give you Ok results, and is readily available. I prefer this to more heavily fertilised composts (e.g. JI #2 or #3) because I find it easier to control the nutrient requirements during flowering. If you are growing guerrilla style you might want to try a different approach though as they will probably be tended infrequently. Seedlings will start fine in this compost too.
The main thing that I would say is to leave the seedlings alone. I think that it is much easier to fuss over them when what they need is just to be left in the sun to get a few leaves going. If you grow any tomatoes then treat them similarly and they will most likely reward you. If you don't then get yourself a couple of tomato plants, you can learn a lot about growing from them, and it helps to disguise other kinds of gardening activities.