Could you please let me know from where are you downloading the en_GB dictionary?? Squiggly is based on Hunspell. Supporting a language implies supporting the Hunspell's rules given in the .aff file of the dictionary. Ideally en_GB should be supported. That depends again on the dictionary you are using. You may download the dictionaries from one of the links mentioned in Squiggly's release notes. The process of installing them for Squiggly is also mentioned in the release notes under the section: "How to install spelling dictionaries in Squiggly"
In case of any issues please let us know.