I've just pulled out the 1996 ATUK as well, and it's as if Cardiff didn't exist - the land that man forgot! It's not even listed in the index. Really strange considering how comprehensive the book normally was in all other regards!
My guess would be that this was a 757 operation though as that was the Britannia fleet standard of the time.
Little-known aspect of Britannia flight numbers: many thought they were random but until you listed out the flights by day of week, the pattern didn't become obvious. BY1 / 8 / 15 / 22 / 29 and so on were Monday numbers, BY 2 / 9 / 16 / 23 / 30 Tuesday and on it went. 198 would therefore be a Tuesday number and indeed 22 October 1996 was a Tuesday (and a "Reus day", back in the era). Useless piece of trivia, but there you go.