The Rent a room scheme was one of the best ideas from the previous or indeed any government and has been around since early 1992 if I remember correctly.
I used it back in the Nineties when I bought my first house and it has become hugely popular with this recession/credit crunch. With the rises in rents over the last 19 years I think the £4250 tax free limit should be raised, room rents in London (Tube Zones 1,2 and 3) must be far more than this now.