Several factors influence the fare charged... The main factor is the national carrier ... Incase of UK British Airways decides the minimum-Maximum range and the 2nd factor is the market competetion. Thats the main 2 reasons for LHR/DOH being cheaper than DOH/LHR. If u check Emirates, Etihad, Gulf, Kuwait Airways you will notice the same pattern. Tip: Always lookout for promotions, some airlines like Air Italia, Air Malta offer very low fares BUT please keep in mind such low fare tickets have all kind of restrictions. Cheers.