Have it by the seaside. In September.
Joking aside, I'm not sure there's enough demand to support two large UK shows and people do seem to prefer Southampton, even though it's more difficult to get to for the majority of the population.
Shows have fundamentally changed now that we buy so much on line and get very competitive prices all the time. It's not just a boat thing, but is pretty much universal. We get "show prices" all the time so are less inclined to shop at shows. Retailers have tighter margins, and sell less when they do go to a show, but the costs are still high. The stand rental is just the start of it, then there's staff, accommodation, and lost production time in some cases.
I liked the outdoors show that they ran alongside the boat show for the last couple of years. It seemed like a good idea, but they've split them this time. Maybe we'll see them come back together next year.