I'm american and go visit all the time. I take empty suitcases out there with me, to fill up on baby clothes, trainers, and of course PC parts. My advice is to ditch the retail box, and hide the reciepts as well as you can. I don't declare what I bring in, (you are allowed to bring in up to £300 worth of goods for personal use btw) and just go through the "nothing to declare line" at the airport. If I have to talk to someone, and they ask if I have anything to declare, I say something like "I delcare that i'm glad to be back in the UK"
A little joke like that goes a long way. Also, if they ask, just say something like "just this bottle of bailey's I bought on the plane."
I've yet to be stopped, and before my daugher was born, both my wife and I had full luggage trollies as we were going through customs, and I had a BOXED pram on top of my luggage as well. lol.
Another thing is, just go with the "it was a gift from my uncle" or something else. As long as you're not smuggling stuff to sell or massive amounts of cigerettes, you'll be ok.
and remember for PC parts: Fry's Electronics.