Where was The Christmas Invasion filmed?

Special episode of Doctor Who

The Christmas Invasion (2005) was filmed in Cardiff, Tower of London, Brentford, Tredegar House and Clearwell Caves, 5 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.

See them on the map → Wikipedia Source data