I'm going over the pond tomorrow DCA-JFK-LHR and then return on Tuesday 5/29. I tried to upgrade with miles going over, but there is only one seat left in UC; however, I was able to score a UC upgrade westbound. The agent at the DM desk told me that she had never done an upgrade like this before, but she was a real trouper and got it accomplished.
The cost was 25,000 SkyMiles, which is a pittance since VS usually wants thousands more for UC than for PE, and since they auction off seats on the day of departure for $800+. There was no additional UK departure tax; apparently the tax for PE and UC is the same, and I had already paid that.
I am very pleased.