Might be better with one of these
http://www.performancebox.co.uk/
No experience with the thing myself but looks handy. No beacons required and can datalog and analyse progress later. Have heard track day organisers are getting wise to these too if used as a timer though. Not sure how the rules stand if you just record with it (ie mount it out of sight and use gps aeriel) and only download once back home - same as a video I would say.
In car lap timers.
I vaguely though about getting a timer device, then decided it was a bad idea as it would only encourage me to push harder and harder for a faster lap which would surely lead to a trip into the tyre wall sooner or later. I find I can usually tell if I am quicker/slower by how I am going relative to some of the KH regulars, and prefer just to focus on being smooth and enjoying myself rather than extracting the last nth of lap time.
Mark
Mark