Do an open jaw BHX-HKG, BKK-BHX and combine it with a single HKG-BKK (just make sure it's a Y flex fare for 1,000 miles or a J Saver for 1,250).
You'll probably find the open jaw fare is less than the the regular BHX-BKK return, and you don't have the extra travel in the wrong direction on your return.