Flights to the UK

Hello,

My girlfriend and myself have come down through Mexico and are now in Guatemala. We have about a month before we have to head back to the UK. The problem is that we don´t have flights back and I´m having trouble finding out prices and working out the best place to fly back from. We could either fly back from guatemala or, ideally, continue to nicaragua and fly back from there, or at a push go to San Jose if it was cheaper from there. But I can´t find out anything useful from the internet. Does anyone have any experience of the cheapest city to fly out from? And the best way to book, i.e. to do it via the internet or to go the city and try and find a travel agent?

Thanks,

Jack.