Dammit! My post was eaten. I would practice damning with faint praise–something the Brits excell at.
Or you could take refuge in obscure American slang–go back to the ‘20’s and start there.
Or you could watch AbFab as a tutorial or BBCAmerica (I dont’ get that channel, but it would give you an intro into the fine art of British insults).
I will say this: the direct attack is never best, especially in a written medium. Go for rapier thrusts, rather than hack jobs.
Good luck. Don’t expect victory, but a recognition of a worthy opponent would be praise indeed.
(I’m American, btw).