I think CWS's advice is spot on, and can possibly be shortened for conciseness to 'don't worry.' I cannot recall the last time I was asked for the credit card used to purchase my ticket, either at LHR or anywhere else for that matter. This has included complicated sectors with bits in the middle changed and reissued, as well as travel during periods when one card expired and another started. This goes as well for travels with MrsLE and out netball-team of children where, for a variety of reasons, different tickets will have been bought with different credit cards.

The last time I do recall being asked for my credit card was in the late 1990s leaving St Barth on a short hop flight to St Maarten. If you've ever been to the former airport you'll know that it's not the height of advanced airport design. Their credit card system (and probably others) had crashed the day before and they were manually checking tickets for security.