Consider accepting beefeater for the guard? I also tried telephone booth for the red telephone box; maybe that isn't specific enough but I had no idea what else you could be going for.
The “Changing of the guard” at Buckingham Palace and Windsor Castle is a well-known spectacle, carried out by soldiers of the Foot Guards. However, since this particular chap is standing beside a sentry box, I would have referred to him as a sentry.