The user confirmed that the workstation locales were set to UK. Also, in OWA, the user confirmed on the Regional tab (Options / See All Options / Settings / Regional) that the language was set to English (United Kingdom).
My colleague has also confirmed that a different deployment has also experienced this issue - this particular deployment is running Exchange 2010 RTM, so clearly it's not unique to Exchange 2010 SP1. It appears to be the result of moving the mailbox from legacy versions of Exchange.
The fix is quite simple. Just set the mailboxes to use the correct language dictionary with this cmdlet:
Set-MailboxSpellingConfiguration -DictionaryLanguage EnglishUnitedKingdom