My parents have a UK visit visa, which expires on November 11, 2018. They have never entered the UK. Is it possible to extend the visa under such circumstances?
There would seem to be no reason to extend a visa whose holder is not inside the UK. Just apply for a new visa in the regular way.
The Immigration Rules App V8 indicates that extending a Visit Visa can only be done while in the UK and in specific circumstances https://www.gov.uk/guidance/immigration-rules/immigration-rules-appendix-v-visitor-rules