In our experience, customs passage in England or Scotland couldn't be much easier. We've gone into both Glasgow and Manchester; customs is basically "Hello, passport please", a brief looking, a stamp and perhaps a few questions as to the purpose of your stay. They always get a smile when you say "vacation" instead of "holiday." The time of course depends on the number of people (and idiots) in line but the customs folks seem well able to cope.

I did get asked once about the contents of my large-ish shoulder bag... I answered "A Nikon, and two lenses." The nice gent asked to see it, which was easy but I asked him why. He said if it looked too new, it might cause a problem on departure as someone might think I had just bought it (and was ducking taxes or duty.) He judged my camera to be OK and so it was...no query on the way out.

Returning to the U.S. of course subjects you to the indignities of the TSA inspections but you're already used to that.

For all the things you should worry about on your trip, I'd says customs isn't one of them.

Go, and enjoy!

JT