Whats a good motorbike for me (read info below)?

I am 14. I want a motorbike to take down the track next to my house. It needs to be an off road bike and i can only spend a maximum of about 6 or 7 hundred pounds. I've been looking at kawasaki motorbikes and yamahas, but lots of them seem to come in kids sizes, WHICH I DO NOT WANT!!! so PLEASE DO NOT suggest anything too small, i want a full size bike, roughly 125cc. I'd really like a minsk, but the chances of geting one in the uk are virtually nil. but if you know where i could get one, please suggest it Please help me!!!!!!!!!!!!!Nothing Over �700 and no kiddie sizes. Please dont suggest a really obscure make that i wont of heard of. stick to Honda, suzuki, ktm, kawasaki,yamaha. Nothing new, i can only affoard used, unless new at a resonable price