In practice, if you're just making a one-off for personal use, you'll probably get away with it for as long as nobody complains about it. If you do get a knock

In practice, if you're just making a one-off for personal use, you'll probably get away with it for as long as nobody complains about it. If you do get a knock on your door, and the agents don't think you're actually trying to be malicious, what will happen is that you'll be told to stop. If you don't stop, then you'll get fined and your equipment will be confiscated. BTW, WiFi frequencies are not "free for all", they're just permissively licensed.