121.5 is an emergency frequency. This was an emergency, and was being correctly used for such communications.
Well, personally I would not call this an "emergency". Sure, within the UK this is a legitimate use of 121.5 but in other parts of the world this would not be appropriate. Instead, you would be contacting the nearest radar/VDF equipped center on their own frequency, and talk to them.
Also, I don't think (based on what I've read here) that DX actually declared a "pan" or "mayday", and used that word consistently in front of her callsign. I don't think there's any pilot in the world who, upon hearing "this is mayday/pan G-DX" on 121.5 would play the guard police, as the word mayday or pan alone would make clear that the use of 121.5 was deliberate and genuine.
As others have said it, the UK is one of the few (perhaps the only) country in the world which allows (even encourages, in some cases) the use of 121.5 for more than just genuine emergencies. It's also one of the few (perhaps the only) country who have a dedicated ground unit (D&D) which monitors this frequency H24. In most other places such a ground unit is not in place and without some sort of informal guard police 121.5 would probably quickly become another sports/chat channel.
Of course airline pilots are supposed to know the local rules, regulations and habits of the countries they're flying in. This is such a local thing. I don't know exactly how airline pilots do things but I've heard that diligent pilots fly with the AIP (or the Jeppessen extract of it) on their lap, looking up local ICAO differences as they fly along. I wonder how prominent these differences wrt. 121.5 are written down in the UK AIP so that airline pilots know about them?