To be fair, the Aces followers do make up a large portion of the GP crowds, easily the largest single contingent...
As you would imagine given the locality...
Therefore I can see why crowds drop, (like last year), in the weeks leading up to the meetings, given the cost..
You also have people like myself, and nine others, who will attend the GP's but are not regular visitors to the NSS, due to, as you say, not having any real engagement for UK Speedway meetings that have little importance..
There are many more "Followers" than regular "Fans", like at most tracks, with the GP's bringing them out...
The key for the UK version of the sport is to re-engage those "Followers" to become "Fans" again, as it would seriously boost the attendances around the country...