I've been in a long distance relationship for about a year now and my girlfriend
I don't think you are wrong to feel as you do, but then she is a free agent and perhaps regards the relationship as more longdistance than a real relationship. The fact that you would not behave like that is not that relevant, you after all do not live in London and perhaps have little opportunity. What is wrong in any case with a bit of jealousy? Are you sure that she did not tell you this to make you jealous and perhaps the event never really happened.
If you care for her you are bound to feel hurt when you contemplate someone else usurping or sharing your relationship with her, unless you regard her as merely one among many who go in and out of your life. At this stage she is really calling on you to define precisely what relationship in fact you have with her and take it from there.