Originally Posted by
TraumaDoc
I'm guessing this may partially be in response to people buying a cheapish fare to/from LGW (which are often much cheaper than LHR/LCY) but changing it to LHR on the day, hence saving a wedge of cash.
I suspect it was the case too, but they do lift the restrictions when there are irrops, and you can go in the other direction (LHR to LGW). In the latter case it seems it's not a "free" move, but since LGW is so much cheaper the fare difference is likely to be below zero anyway.