Yes, not sure if you want to be a professional pilot or not but try and do some good and long cross countries. That is a good way of building your confidence and experience. Also, as people suggest try and do some night flying and try and head down to France or Spain for a bit of a jolly - you'll learn a huge amount by stretching yourelf.
If you can go overseas, I would spend a few weeks on the West Coast - doing a bit of high altitude flying, operating at hot n' high airfields etc.....all good learning.
If you're going to get additional ratings and intend to remain as a GA pilot, don't bother getting ratings that you're not going to use. A multi engine rating is great but like most people, you'll probably never use it in the UK as the cost is prohibitive and you'll probably not fly light twins often enough to keep current. Likewise, if you're planning to fly IMC a lot then go for the rating, but if you don't use it you'll get rusty pretty fast. Still, it is useful to get you out of trouble.
Hufty.