Hi. I understand that immigration in Chaengwattana frowns upon renewing volunteer visa which were approved locally. However if the volunteer visa was given by the Thai Embassy in the UK would that be a different story?

It certainly would not be a problem getting a extension of stay based upon working as a volunteer if you entered the country with a non-o visa issued by the embassy in London.

This from the embassy website. You would apply for the visa on the e visa site.

Source: https://london.thaiembassy.org/en/publicservice/84508-non-immigrant-visas?page=5d6636cd15e39c3bd00072dd&menu=5f4b6eb3f6ae4b236972c562

You could also apply for a legit non-o visa at immigration if working with an organization that can provide the correct supporting documents. See: https://www.immigration.go.th/wp-content/uploads/2022/02/10.FOR-WORKING-IN-THE-FOUNDATION-NON-O.pdf