pgh234 is correct. OpUps used to be more common, but airlines in general, and DL in particular, have gotten VERY good at managing seat inventory and filling TATL flights just up to capacity but not over. For now, think of it like holding a winning lottery ticket - it does happen but not often and probably not to you.

Having said that, there are still factors that can increase your odds of getting a rare OpUp to/from London. Do not fly on routes like LHR-JFK or LHR-ATL where DL routinely completely sells the J cabin. Do fly on a route like LHR-MSP where there are lots of leisure pax (increasing the opportunities for an oversold flight). A few years back, as a PM, I got 5 OpUps in one year out of 10 TATL flights, all between MSP & LHR.