Antenna rubber ring thing

I decided to give my car a bath and while doing so, some of the rubber ring around the base of the antenna broke off. The rest of the ring is quite brittle but I left it on there because I'm not sure what kind of damage would occur. Is there a way I can replace this? What part is this specifically? Again, it's the rubbing ring at the base of the antenna. I have a 2007 cooper S.

Thank you!