Do you have a UK Visitors Shotgun Permit?
Just taking a guess, since you'll have the guns in your posession while you're in the country, you need one to be perfectly legal.
That is unless there's some purchase loophole that I'm unaware of.
I don't think they'll give you much trouble at Heathrow, but have your paper in order when you come back into the US. Print out whatever you can find that lists the serial number range on the gun so you can prove it's an antique and have your bill of sale right handy as well.
As far as import paperwork I'd start trying to call the goverment folks and ask questions. It's usually time consuming to get a straight answer from the ATF. I've exported guns before but never brought one back in unless I took it out with me as well.
Destry
