This is available with the Ordnance Survey Opendata Boundary Line dataset.
see BoundaryLine (Download as Shapefile)
Note: It is worth looking at the start guide for more information on this dataset.
OS OpenData getting started guide (page 12)
Boundary-Line is a vector digital mapping product that is a complete set of local government administrative boundaries and electoral boundaries used in local and general election voting. It has been specifically designed to show the area of each administrative or electoral boundary.
The product contains the following boundaries:
Civil parishes and communities.
Counties, Districts, Wards, Metropolitan districts, Electoral divisions.
Greater London Authorities and Assembly, London boroughs, Metropolitan districts, Unitary authorities.
Westminster parliamentary constituencies, Scottish parliamentary constituency and Electoral region,
Welsh Assembly constituency and Electoral region.
European electoral region.
Mean high water mark and Census agency codes.
Print: 1:10 000 scale recommended
Screen: 1:5000 – 1:8000
Great Britain (England, Scotland and Wales)