The autocomplete is offering me words which I would prefer not to be in the dictionary. However, I cannot find where to edit to remove them. I have tried enGB but the relevant word is not there. I have entered it in the section of ignore words to no effect.
my understanding is: Word Completion does not use words from any dictionary or thesaurus but collects the words from your current document(s) you edited during a LibreOffice session (depending on setting of option
[x] When closing a document, remove the words collected from it from the list - path to configuration item: see next paragraph).
Hence you would need to Delete words from the Collected Words list at
Tools -> AutoCorrect -> AutoCorrect Options...-> Tab: Word Completion. But that wouldn’t be worth any effort since the list is deleted anyway when closing LibreOffice.
Ref.: LibreOffice Help - Fine-Tuning the Word Completion for Text Documents (See Scope of Selected Words)