You can create your own from:
Supply format: CSV
Code-Point Open provides a precise geographic location for each
postcode unit in Great Britain. The product is a CSV file containing
postcodes, grid references, NHS® health and regional health authority
codes, administrative ward, district, county and country area codes.
Some of this data is in WGS84 on http://parlvid.mysociety.org/os/
GB Postcode Areas
or use the pre-converted (check the age of this source)
Thiessen polygons were generated from all records in OS OpenData
Code-Point Open. Resulting coverage was dissolved up to postcode area
level and then clipped to GB extent of the realm using OS OpenData
Boundary-Line. Names of postcode areas were added from Wikipedia.