I'm not there yet, in that I haven't finished the CPL, but I have my eye on the instructor course at Curtis Aviation, in Camden. A good few of my friends have completed the course, and though many say it's the hardest thing they've ever done, they all commend it. Training is conducted in the Citabrias, and Craig Marshall has an excellent reputation as a teacher (and is a top bloke, too).
Hope this helps,
GWAS