A few years ago (2019) I bought two of those cheap, clear umbrellas in Japan. They were too long to fit in my suitcase so I carried them.

I was told I'm only allowed one umbrella, so I taped them both together and was let through without issue. These are relatively flimsy though, so would no doubt disintegrate should I have attempted to hit someone with them. A beach umbrella is something else!

The flights were ITM - NRT - LHR - BSL, although I met someone during my LHR transit and gave them one of the umbrellas.