Let me correct that for you.
I'm quite capable of organising my own itinerary, but if I go to a show I go expecting it to be worth a day of my time. This is likely if there are talks/workshops etc.
Outside the M25 venue costs crash so even somewhere like Manchester or Birmingham would be cheap enough to drop the exhibitor fee, get more exhibitors and make a similar profit(or more). Although some 'premier' exhibitors may be less willing to travel.
The choice ultimately becomes between small show in London where you need extra-curricular activities to make it worth going, versus a bigger, more diverse show outside London that might be able to keep you engaged all day.
I have been involved with various exhibitions (not musical) over the last six years. They have all be hit hard by the internet meaning people go to look rather than buy. The ones that are succeeding outside London do so by maximising the display/lecture element AND attracting lots of smaller traders.
London's advantage is a ready audience on the doorstep with minimal travel costs, but expensive and time consuming for anyone from further afield. London-based people are less likely to travel elsewhere (after all the rest of the UK is shite, isn't it?) but places like Bristol, Birmingham and Manchester have big catchments from the rest of the UK.
Hmm ... might take a look at the Merseyside Guitar Show, haven't been to Liverpool for a few years.