You can use it for 6 months in a 12 month period. I would imagine the 12 month period starts on the first day you bring the scooter over here, so if you use it for 6 months, you need to return it for another 6 months before you can use it again without registering. If you are stopped by the police, you may need to show when you first brought it here, and so on to prove you haven't had it here for more than 6 months.
(Original post by gbduo)
Ask the DVLA. I doubt they would notice anyhow tbh.
With the amount of ANPR cameras and the likes around these days, I imagine they could find out fairly easily.