The pity here is that OP did not call BA from DXB. BA has a uniquely consumer-friendly provision in its COC which commits, not to rebooking, but to determining what can be done. Perhaps here, BA would have simply changed the LHR-GLA, imposed a fee or collected only the fare difference. But, if you don't call, you have simply no showed and the consequence of that is clear.
Thanks for that. In Dubai I didn't have access to a phone but was able to send an email to BA to advise them that I couldn't make the flight, as I thought that would be better (and more courteous) than simply not showing. BA replied to my email three weeks later (!) as follows:
Thank you for your email dated 28 July 2014. I apologise for the delay in replying to you. I am sorry to read that you could not travel as planned. For any applicable refund of your unused ticket, I would request you to contact your travel agent. As you are booked through them, they are in a better place to deal with this matter. Once again, thank you for contacting us. We look forward to welcoming you on board soon.
Just being able to vent on this site is making me feel ever so slightly better, but still no further forward in getting any of my money back :-(