I think you have made the right decision to go ahead. I hope the following observations from five years ago when the Calne show (in January) was affected by bad weather, will be helpful.
We went ahead, despite snow lying on the ground. The main problem then, as seems to be the case now, was access to the main road network from the smaller roads. That meant some of the layouts could not travel because they could not be removed from the premises where they were stored. The trunk network remained clear.
All of the traders still came.
One third of the layouts did not, but most of the visitors were very understandng, accepted the gaps, and praised us for going ahead.
Attendance was down by 65% compared the the previous year.
But - we still more than covered our costs.
My analysis after the show revealed that, had we cancelled, we would have lost money. This was due to the up front costs such as publicity which could not have been recouped.
One important lesson we learned was the need for widespread publicity, especially through the local media, that the show is going ahead. You will also need to ensure that someone is available to answer the main contact telephone because people will ring up and, if they do not receive an answer (an answering machine will not do) they will assume that the show has been cancelled.
Good luck!
Geoff Endacott
P.S. Except, of course, it has now obviously got a lot worse and without access to the venue you had no choice.