Lucky you!
I think 'Plane Euclidean Geometry' and 'New Problems in Euclidean Geometry' would be good if you were planning to compete in the second round of the BMO, but they're probably extremely hard to read.
'The Mathematical Olympiad Handbook' has a large range of questions and guides you through them if you get stuck. That said, it's for reference only at my school's library so I haven't been able to look at it in detail. Again, the questions are on the hard side. I'd suggest going to a library and having a look at the books on Euclidean geometry. That way you won't end up buying a book that you won't understand.
I'm still considering applying for Maths with Physics but I'm not 100% sure. What do you do in the first year for the Physics part of the course at Cambridge?
You still have absolutely ages to decide which college to apply for so don't worry about that! Besides, it doesn't really matter in the end.