I still have around 100 domains in that account not 60, but I have transferred around 90 domains from the account in the last few days. So once you reach 100 domains the tick boxes show up but the other option is still available.
I think what Caz meant was when you reach 100 or less domains in your account then the check boxes show up. I have recently transferred around 90 domains away from my account and I think that's why the check boxes are showing up. Its a useful feature.
It was my mistake thinking it was a new feature because I normally have a few hundred domains in my Nominet account, I now have less then 100 domains in my account.