You need simply rise with a flourish, shoot a confident look around you and say, “Yeah? Well, the jerk store called and they’re out of you!”
If your nemesis is able to apply an equally witty retort, your next line should be, “Well I said sex with your wife!”
This is as fool-proof as it gets and you simply can’t miss with these gems.
A cute little gem I got from a book is “Your mother won the horse fair and your father tracks rabbits by scent.” The classic Latin anglicization “Fac te” works too. That is, assuming I remember it right, my Latin’s more than a little rusty.
One I’ve used a few times is to shake my head and say, “To think I could have been your father if I’d only had change for a quarter.”
(Okay, that only works if you’re a guy.)
Since he’s British, why not try, “Your father was a Nazi collaborator and your mother swam out to great the troop ships?” It’s a bit dated, but it might work.
“In the breadbox of life, you are a crumb.”
I also like a casual, flippant “fuck off.” You have to say it with a blend of disgust and carelessness. Say it as if you are just too exhausted and not at all interested in anything else he has to say. Then just walk away, or open a book. He will be flabbergasted and embarrassed.
It’s worked many, mnay times for me.
My brother has always told me that the best way to insult men is to use female epithets like “cunt” or “bitch,” but I’ve never tried it.