Thanks for the reply - sadly no APD exemption for EXT, I just happen to live 20mins away! I think I have now worked out this is an MCT issue (or at least an Emirates web-site connection time issue!) The outbound (EXT->GLA->DXB) is a 75 minute connection, which the Emirates web-site refuses to offer (though it does offer the 25h15m connection to the following day!) On the return, DXB->NCL->EXT (2h55 connection) is bookable.

Domestic to International MCT at GLA seems to be 45mins, so it appears to be an Emirates web-site problem - will call them and see if I can book over the phone.