GB english spellchecker in Opera 10 on Linux

Opera 10 has added an inline spell checker which checks spelling in web forms, but the default (and only) dictionary in the Linux version is US English. Here is how to add GB English, and other languages.
Curiously, the Windows version already has a GB English dictionary option.
UPDATE: This post is now obsolete (and the link above dead). The current version of Opera has a menu option to add spell check dictionaries.