From 1 April 2013 the Health Protection Agency is part of Public Health England (PHE) [external link].
This page provides introductory information about many varieties of radiation, with different levels of detail:
At a Glance: engaging online presentations (requires Adobe Flash player)
Frequently Asked Questions: information presented in question and answer format
Information sheets: Introductions to topics, about 1-5 pages long, with some illustrations and references
Understanding radiation - topics: detailed outlines of topics, with links to publications and statements