What’s a good response to “good for you” (sarcastic)?

Occasionally I’ll be discussing some small personal achievement (such as completing a difficult wall climb at the rock climbing club, or finishing a video game) and been met with a sarcastic “oh good for you”.

It’s usually a third wheel as well, as in I’ll be chatting to a friend or co-worker and someone else who I don’t really talk to is constantly butting in with a sarcastic reply.

Doesn’t happen a lot, but here in the U.K. there’s a national pride in keeping others down and in their place. Would be great to have an insulting reply that stays within the grounds of decency to put their manners back in.

Thanks