whatever class you are travelling, you can opt for coach for your "extra" segment, and of course, pick the shortest segment as 'coach'

in your case, e.g., say you fly D/A class, you can pick the shortest segment, i.e. bkk-sin or sin-dps(not sure if there is OW flight on this route) in coach. so that you only have to add GBP75

and, if you are flying D/A. CX is significantly better than AA. if flying coach, perhaps the seat pitch becomes more important, and if you are over 5'5", go AA