Try this website. Its an application running from google maps so is very accurate.
http://www.gmap-pedometer.com/
Before starting select options and choose uk from the top drop down menu. Then type the place , town or city you want. Once you have the map at a suitable zoom level to see what you're doing choose start recording on the left of the page. Now double click on the map to mark the points you need to pass through it should display pushpins on the map as you go along and a totla distace will appear in the box on the left. There is also an option (complete) for a return path along the same route.