So, is it illegal? Nope. You’re free to fly the Union Jack. Just follow basic decency: don’t hang it sideways, don’t stick it on a toilet, and don’t use it as a napkin.
The law is on your side. The flag nerds are watching. But you? You’re having fun.
Go on. Hoist it high. And if someone asks if it’s legal, tell them: only if you’re brave enough. (You are.)