As Jerry a. laska mentioned being UK based, it may suit your travel plans to use LHR as the place to take time off from flying. If you start in DOH you pretty much have to return to UK and then once you leave the UK you won't be back until the ticket ends. (This presume you would not want to go to Asia immediately after DOH)

Whereas if you purchase in Asia - you can do a flight or two in Asia plus a flight back to LHR (hang around as long as you want) A separate holiday to Dubai (or MCT,DOH,BAH etc) and back after that
(plus MCT or DOH run bought additionally). Then a separate holiday or trip to Europe (hang around for as long as you want) and then onto the USA (finishing in Asia).

Of course after dealing with LHR that much will mean you need as long and as many holidays as you can get. I realize from what you say you aren't over interested in extra trips in Europe but sometimes being able to split the ticket into smaller trips can find value that goes beyond the difference in price at different starting locations .