From my quick peruse of the JARs, I can't find any way of getting an instructor rating valid for use within the JAA without doing the whole instructor course ie about 30 hrs flying etc
I have 2300 or 2400 hrs instructing from ab to multi, IR, instructor & aero. I'd like a UK instructor rating but don't think I'll bother if I have to do a 30hr course before I can do the test!