One of the first things you might notice about Heathrow Airport as you apply through their career site is that they advertise their pay ranges for some positions and don't advertise them for others. If they view you as a potential candidate for an offer during your time with them, your interviewer may pose this question to ensure that you would be willing and ready to accept their offer. It is best to be honest as you answer this question, but you also want to ensure that you've done your homework on the position and don't price yourself out of contention. If you need to, you can certainly ask a clarifying question here.