I can't imagine why you would want to do this since OW Explorer tickets (I asume this is what you are talking about) are so much more expensive in the UK than in Europe or in Cairo.

But on the one or two occasions I have needed to use them (normally to help out their colleagues in France who are fairly clueless) I have always found the Cathay ticketing desk in London has been very competent and helpful on this type of ticket.