you know what!, Iwas thinking that too last night, aside from the example you gave, they will save about $180 per person, so $360 for the pair
given the effor required to catch another plane which means more costs getting to the aiport, and the time on a train, is it worth it???
as you said unless they reallty want to go to paris, paris definitely is a preference for them, but its not essential,
I was thinking they could do a return trip straight to LHR, but extend the singapore stop over for a few days instead,
but now Im definitely going to offer them the option to go straight to and from LHR even if it will cost the extra
but great suggestion, I have read on these forums people changing airports to save a few dollars and my initial feeling was "geez that seems like a lot of hard work "