I wonder what the railway equivalent is...
£1 billion?
On a more serious note, I don't think spending money to make roads more dangerous* is a good idea.
*New roads need to be 3.65m wide per lane (although it is possible reduce this); there must be some good reason for this standard (although I'm struggling to think of any myself), otherwise why would we spend a fortune building new roads to this standard? So narrowing existing roads doesn't seem a particularly logical suggestion from this perspective.
Having said that, I'd be in favour of a "B road standard" (not as wide, sharper curves allowed etc) as well as the current DMRB** standard in order to reduce costs. I've never come across one if it already exists and I doubt one would ever be created.
**Design Manual for Roads and Bridges