The set and prop designer, model maker Andrew Glazebrook is quoted that, "Its registration number XH558 is actually that of the Royal Air Force's 'Avro Vulcan' bomber and was suggested by the show's writers, Andy Riley and Kevin Cecil."

So much for my eagle eyes.

Also: read this page for details of brake testing procedure during development. Rather than mount cameras on the plane, they strapped a photographer to the front gear strut. Interesting risk assessment exercise.