The £58 pound flight is when booked as a return flight LON-LUX-LON. In that case the return leg would also be about £58, so the total would be £116. I saw the E19 single flight LUX-LHR. I got the feeling you just discovered an error fare. Do yourself a favor and book a one way ticket with EZY one day and a £16 BA ticket for the return leg in that period and enjoy a nice weekend in the Grand Duchy. I don't think you'll find any cheaper fares soon.
For what I derived from your first post, you have one itinerary with SAS that is LUX-CPH-PVG-CPH-LON, which makes your journey an open jaw. And now you just need a way to get from LON to LUX on the cheap. So I checked one way fares. In that case BA now has just one ticket left for £89 one way. Luxair has a seat for E124,5, so basically about the same amount of money. One way fares are more expensive with most airlines than return tickets. It's stupid, but it's the way it is.

EZY is not very likely moving this flight to another slot. I wouldn't count on it.