A pair of dictionaries.
Diacritics allows fingerspelling of letters with diacritics, mostly using a shape-based system.
British Words allows you write most of the words listed on the Wikipedia page 'Glossary of British terms not widely used in the United States'. It doesn't include British spellings for words already in the dictionary (I recommend Ted's UK Spellings dictionary for that). A couple of entries from the main dictionary have been overwritten, with new entries added where needed.
Update: Some British pronuncations, sourced from 'American and British English pronunciation differences' have been added now too.