If you want to bump, then request it at check-in or the ticket desk on first arrival at LHR. It has worked for me lots of times. Remember to have an alternative plan for your travels. In addition, they may not want you to check-in and send the bags to security at that point. You may have to wait landside.

However, it is a very good way to endear yourself to the staff and I have had a 60% op-up rate when given a seat on aircraft I knew were full. First for deeply discounted BC, you never know. However, outstation op-ups are more likely than ex LHRs with this.