There's no set rule as such. I mean, like jim said, they're not as harsh as they are about maths. But before you hand in your form, you have to go to your 2u and 3u teachers and see what they think. But generally no one even checks if you've gone to see your teachers and if you don't go and see them, they won't even know. You could pretty much get away with anything. It's always best to check with them though - and they're not like maths teachers, they don't base saying 'you should/n't do 4u' just on your ranks, they look at you as a person, how you've been going through the year, how hard you work, enthusiasm, etc etc. They let the guy who I'm pretty sure came last in 2u english take up 4u, so if he can do it, I guess anyone can =)
How's it going on the maths front?