...assuming anyone is even there to bother about it. Unlike the US, say, UK customs operations are very targeted, so you'll often find they're not present at all. This is particularly the case at smaller airports - at BHX T2 at last count the "red channel" was just a telephone!

I imagine the reason for it is that you might have bought duty-free then be reimporting it.

That said, at Edinburgh there is a direct channel to domestic arrivals from the departure lounge - just follow the signs for Arrivals (at some gates, domestic arrivals go straight into the terminal). No need to pass any customs there.

Neil