I have found that flying to Belfast is relatively inexpensive. I had a return ticket through there last May/June and it was $450 after taxes on Continental from Newark, NJ. Then I just got an EasyJet flight from there to wherever else I was going in the UK for £10-£30, can't recall precisely.
Currently looking at one-way tickets from the US through there and the price is $575, after taxes for the same time of year. All other one-way tickets to Glasgow, Edinburgh, London, Manchester, etc. are at least $1000.
It seems like it's as cheap to fly through as Dublin, but you don't have to worry about the whole "Dublin Loophole" stuff because of course it's the UK.