Well I'd already paid my miles for one way to JNB in F and returning paid J to LHR. There was one F award seat from LHR to DXB on EK008 on my date and I've paid my $25 fee to change. Not particularly stressed but as it's a short holiday trip to take more safari photos I'm happier with a guaranteed F seat. If there was a downgrade from BHX I'm sure that any discussions about refund / compensation would not be completely smooth and I can't be bothered to argue. Also while J would be fine there would be a fair chance of ending up in a middle J seat on the 777 as I would be pretty much the last to choose. Thanks for all the input and advice and hope that F from BHX turns out to be more popular than it currently appears.