As reported recently in the Dorset Echo.
The GDSF managers explained that this is the time of year they would start booking the site facilities and contractors, (presumably as well as taking exhibitor bookings), but even though the event is in August, the outlook for public events this year is so unpredictable it was thought better to cancel.
Elsewhere in Dorset: Swanage Railway will be re-opening services. I don't know about its allied Clay Mining Museum, adjacent to the SR's Norden park-&-ride station.
I have not yet heard if the Chickerell Steam & Vintage Show (near Weymouth) will go ahead. Its normal weekend is in early July, and it is a very miniature-friendly rally.