Umm, I dunno, not even sure if there is one? Anyway, K5 owner here too, with an aftermarket 'xorst and obviously there were no probs whatsoever come MOT time, it's a road legal exhaust, stamped as such. I did put the baffles back in just so as not to take the wee-wee but I suspect knowing my shop they'd have happily passed it without baffles too.
EDIT: Sorry, just adding more questions! Is there even an emissions test for bikes at the MOT? There have to be a few old skool 2 strokes still kicking around, can't see an old RG250 ever passing emissions?
Suppose there's a bit of room in the manifold at the 2-1 junction where it might be possible to squeeze a small cat?
Hmm, you've got me thinking now