It's an offence of sorts to sublet ANY house you rent without the landlord's consent.
The reality though is if you do sublet your house, you won't go to prison for it, and it's hard for the landlord to actually take as much action as he should probably be entitled to.
At the end of the day it's illegal and undesirable because the landlord has the right to know who is occupying HIS property. But then again, everyone in this day and age has a right to a roof over their head, don't they?
It could also be said that subletting a council house is a worse offence than subletting a private rental because council houses are social, rather than business, projects, so it stands to reason that no-one should make money from them.
I'm torn if I'm honest. Surely there should be one rule across the board, and what applies to council property should apply also to private rented property. But given that the two serve essentially different purposes, it could also be said that any risk you take with buying property to rent out should also extend to taking a risk on who occupies it.