I don't know about the 1980s but I have very good memories of The London Music Shows held at Wembley in the 90s. They were very NAMM-like, in that they catered for all musical instruments, sprawled throughout several buldings, and had excellent star guests. Tickets were cheap even by the standards of the time, and included quite a lot of freebies. Very much unlike the LBGS, the Music Show had among the exhibitors a lot of big names with global market presence and practically no small luthiers.
IIRC the rot set in when the Music Show was moved to the Excel in East London, a much less suitable place for that kind of event, and then was killed off by the 2008 recession.