How cold you find Scotland, summer or winter, depends on where you come from in the first place. As a former New Englander, I rarely find winter weather in Scotland bitterly cold. Given how far north this part of the UK is, the climate is temperate.
Not everyone would agree, of course, so this chart of average temperatures in Scotland should give you an idea of what to expect.
What can make Scotland seem colder than the mercury would suggest, in spring and autumn, are the preponderance of older houses with thick stone walls and big drafty windows that tourists seem to pick (real Scots have cottoned on to insulation and good central heating just like everyone else - it's those mad visitors who like the drafty old castles).
If someone offers you a "hottie" before bedtime, they aren't suggesting any between-the-sheets shenanigans. It's what the locals call hot water bottles and they are invaluable in big old country houses to make the beds easier to climb into. Even in August.