I would only say that the UK market is declining or will decline in relative terms to the expected rise in some regions like Asia where burgeoning economies will provoke a much keener interest from the major players and so possibly would reduce the exhibition spend in more mature markets like the UK.
I think it would be going too far to say that the UK market is no longer important to some of the major players but it will lose some of its relative appeal. I believe that since the IMHX (and its predecessors') heydays several decades ago there has been a steady decline in attendance from the public partly because certain trade shows are difficult to justify the public's attendance costs, given the ease of information access thanks to the net. Of course, the steady emasculation of British manufacturing industry has not helped either in terms of exhibitor numbers.
One cannot help having the feeling that these big trade shows are put on more for the benefit of the organisers and so fail the first law of marketing, which is give the consumer what the consumer wants rather than what you think is best for them. Perhaps the way to go is to have smaller, much more highly focussed shows which are much cheaper to exhibit at and perhaps much more useful to visitors.