Find your local Evolution Chips dealer. For £199 that should see you a shade over the 100 mark with one of their remaps and for your budget you cold easily get a nice rear exhaust and K&N Panel filter first to improve the breathing a little and make it sound nice.

Don't think there's any point in spending any more money beyond that on that engine. Some uprated brake pads and Eibach springs would be nice to improve overall performance and feel on the road.