The Croquet England website has much useful information, including sections about two versions of the game, the traditional Association Croquet ('AC') and the newer Golf Croquet ('GC'), and about clubs in England and Wales, tournaments, equipment and archives. It is affiliated to the World Croquet Federation. The national federation is divided into regions, and the Watford Club is part of the East Anglian Croquet Federation.
Croquet is played in several other countries, in some cases under variants of the AC rules and GC rules, notably in
Hong Kong
Mexico
Portugal
Uruguay
See also the World Croquet Federation.
Some of the Watford club's members are listed in the UK and Ireland rankings and in the World rankings.
There is a forum for discussion of issues about croquet at the Nottingham List.
Croquet World is an online magazine.
You can find extensive information about the game on the excellent Oxford Croquet website.
There are beginner's guides to playing croquet at Garden Croquet and Playground Equipment.
If you are interested in watching croquet played at the top level there are now many videos available on YouTube; just search for 'croquet'. In particular you can see a short video introduction produced by Croquet England.