if u take a stop in doha on the way to dubai, u get charged for taking that stop. if u decide to take stops on both diretions, u pay for each stop. if its a very cheap ticket, u cannot stop in doha and if u do, u will need to pay additional charges that equals to a price of a london-doha ticket plus other charges. if u do not use it in the right sequence, the whole ticket gets cancelled and probably no refund is given.

its best to just get a london-doha ticket ...