Yes this is indeed Brian. This is taken from the youtube video description:
{Very brief clip of Brian Setzer at a party hosted by Richard Branson at The Manor, circa 1980. XTC's "Generals and Majors" plays in the background. Looks like Andy Partridge taking a splash in the pond. Don't expect any sound bytes from Brian, and in fact, the other Stray Cats are not in the clip, but it's cool to see Brian in his pre-MTV clothes and hairstyle with a plate full of food. From the last reel of the XTC documentary "At the Manor" from 1980. }
I think, but am not 100% sure, that this was a party for Virgin Records, Branson's label. I believe the Stray Cats were invited b/c of playing in and around London at the time and were guests of Keith Richards and the Rolling Stones who were signed to Vrigin Records. The Stones were instrumental in helping out the Cats when they were starting out, as has been documented in other interviews, etc., after Richards and(?) Jagger saw them perform at a nightclub.