List of UK hill climbs
You can use the Strava website to find categorised segments. You'll have to register for an account and I'm not sure how "official" the categories are.


Go to Strava.com 
Register or sign in Click on "Explore" menu
Find the region you're interested in using the search box. 
The map will then show all segments registered in Strava.
There is a slider along the top that allows you to filter out various categories, so you can see just categorised climbs.