The solution, as the story goes, was simple. And by "simple," we mean "cartoonish and insane." To convince the higher-ups that they could use an illusionist on the battlefield, he would float a fake but realistic version of the German battleship Admiral Graf Spee down the Thames. Just so we're clear, this wasn't a little tugboat that Maskelyne whipped together in his backyard. The Graf Spee was a 610-foot long juggernaut ...
... that had been scuttled the year before. So when Maskelyne created the illusion of the floating ship using a small balloon model and mirrors, his superiors were duly impressed. Because as much as the Allied forces wanted to defeat Hitler with their massive armament, what they really wanted to do was not have to fight Hitler because of their massive armament. A guy like Jasper Maskelyne could probably do some damage in that department.
Now, here is where we have to note that there are no surviving photographs of this balloon battleship (to be fair, there wouldn't necessarily be any even if it were true), so there are people who doubt that the illusion was actually pulled off. If not, whatever he did do persuaded the higher-ups to let him take his act to the battlefield. In other words, the man either showed them some mind-blowing magic or gave them an equally impressive line of bullshit.
"For my next trick, I'll turn her ass into a battleship."