I did ORD-EDI in 2017 [July 25th, UA118, 757-200 ship N14121, seat 4F if we want to be specific] as part of one of my more epic journeys -- my wife and I were married at a Eilean Donan Castle in the Highlands (turned out to be significantly less costly than a hotel ballroom in Cleveland and way better scenery) and because we did it the hard way by legally getting married in Scotland and due to a slight miscommunication it turned out the registrar needed to see the originals of some paperwork so me flying to EDI, highly confusing an immigration official, hopping in a rental car for 4 hours, dealing with the formalities, driving back 4 hours, and hopping a flight back to the US was the only way to guarantee the deadline would be met (In the process I discovered Hertz UK doesn't provide spare t(i/y)res, and met both one of the nicest customs officials I've encountered worldwide and two of the nicest tow truck drivers in Scotland)

It would be nice to see more lift to Scotland but I'm not sure there's the traffic to support it year-round and UA has such density with LHR that it's not that difficult to get there without dedicated nonstops so in an aircraft and gate constrained schedule I'd not be surprised to see UA focus on more difficult-to-reach destinations, but what those might be I'm not sure of (South America anyone?)