Glad I'm not the only one finding this irritating! As someone pointed out, I have also used the method of 'Book with money and upgrade with Avios' to find award availability on DXB-LHR vv. Admittedly, there is sparse availability - but occasional dates are possible to book. This method works if you're looking for specific dates only but less so if you're scanning a range of dates.

BA IT in a nutshell - some things just seems to be too hard to get right..