In my experience (working for the highways info part of a council) if you can get another council employee to ask on your behalf (from a different section) you can usually get the data for free, or try to go through the councils cycling officer. If you pay the fee (usually about £39) they can supply a plan for a small area (3 or 4 streets, 1:1250 on an A4 sheet) which will show the exact boundaries of the public highway. They won't hold data on private land though, just land owned (or maintained) by the council.
Can you be my council employee? :)