I understand why bussing you to DUB to get on the diverted aircraft would be the a better option for you (and the other WAT-BHX pax), however, the diverted aircraft was almost certainly needed back in BHX ASAP to avoid delaying many more pax on the other routes the aircraft was due to fly that day. It's a choice between inconveniencing the passengers on one flight or inconveniencing the passengers on five flights later in the day. From a commercial stand point, I think the right decision was taken.
Also, I think you may find that the WAT-BHX sector is the crew's 4th sector of the day from an early start, so delaying them on the ground at DUB for a few hours to wait for pax to be bussed from WAT may well have caused issues with their duty hours.
These things are rarely as simple or straight forward as they appear.