There are a couple of problems with taking it:

- Likely/possibly not being allowed through security and having to check it in

- If it gets through security, it is likely to be taken off you because it is oversized, and BA.com tells you to check it in

- It might be allowed unless you have a larger hand luggage and a small piece that fits under the seat in front of you, but it is not very likely on a busy-ish flight, and likely to have to give up either the large case or the racket

- Likely not having a sturdy enough case for it due to having planned to take it in the cabin

- Possible damage to it due to not having check it in an appropriate case

- If it does get damaged, not having been checked in in an appropriate case and therefore there may be no insurance or airline coverage

Any way I look at it, it's not worth the risk.

Trying to save maybe 15 minutes could cause you to spend more time getting replacement/repairs and could cost you more than the money you earn in 15 minutes anyway.