You will get attention from the Law and they'll start sniffing round how long you've been here. You may have nothing to hide, but it would be better not to stick out like a sore thumb.
Plus you've got to transport it here.......and what happens when the MOT runs out? Not sure about insurance these days.....isn't it limited to 90 days or something?
There used to be a huge disparity in pricing between UK and Spain......bikes were much more expensive here. From what I hear it's not like that anymore.