what i did was to ask the misses to go to a travel agents in the phils, ask about one way tickets to the uk, we picked a airline and flight, paid a deposit, i sent her the money to buy the ticket, she told me what airline and the flight number, i then looked around getting the cheapest return flight returning on the same flight as the misses, and phone the airline to reserve a seat next to hers . no problems - thou you would probably need to book early as there might not be many seats left on earlier flights. probably not an ideal way, possibility i couldn't get a ticket to return on same flight as hers.
thou i think it could be possible for you to buy a return ticket and then a oneway ticket to the uk on-line, one after another, but maybe others have done that on here ?