in my limited experience with scottish weather, I find it more intimidating than anything else. The place is exposed, not many trees to offer shelter, it's often windy, rainy, dark cloudy... it feels inhospitable but I don't think there's anything there that prevents experienced ultralighters to apply the same principles and gear and be fine. If it can be done in, say, Alaska, I'm sure it can be done in Scotland.
I've been in the western highlands for a week with a tarp in late march (but I wasn't camping every night), big tarp, no bivy. The weather wasn't too bad but I had my share of wind, cold and wet. I was fine. No big deal even for a beginner ultralighter back then.
I think it's tradition that strongly drives people's likes and dislikes about gear choices in each locale. If heavyweight is the stantard, you'll have to give lots of explanations if you go lightweight but that doesn't mean it can't be done or it's not safe. It's happened to me in several other places where heavyweight is the standard, no matter how severe the weather patterns were (more or less than in Scotland, I've tried both).
Incidentally, I'll be back in Scotland late april / early may for another week in the highlands, just before the challengers get there and also in an west to east direction. I'll go lightweight but I'll take a tent this time (Stephensons 2C). It's not that I don't trust a tarp but I also appreciate the flexibility of camping wherever, which I consider an important part of the experience for this trip and I don't want my shelter to be the weak link that sends me off that magic location because it's gonna be windy. Ok, I'm a wimp ULer and Scotland feels intimidating after all :)