Where was Twelfth Night filmed?
1996 film by Trevor Nunn
Twelfth Night (1996) was filmed in St Michael's Mount, Padstow and Lanhydrock House, 3 places named in its Wikipedia article, in the United Kingdom. Wikidata records no filming location for it at all, so the article is the only recorded source. Each place says where its claim comes from.
Places per Wikipedia's article
The Wikipedia article names 3 places. Each links to the sentence that says so.

