the best thing to do would be to view potential places prior to moving in - or ask the landlord when you view them if he can fit locks - at the end of the day he'll be making more money from you than the cost of a simple lock.
Some private flats have locks, some don't. I guess it's the norm that students moving into private accommodation will know each other so locks aren't really necessary. I imagine that if you have separate tenancies the landlord is probably required to have locks on the doors. But usually it'll be a joint tenancy so it might have locks but it might not. Apparently it used to be a law that you couldn't have locks if it was a joint tenancy but that has now changed.