As far as I'm aware, straight TACC never prevents you setting a speed above the speed limit. When autosteer is enabled, however, the car will sometimes decide to restrict the set speed if it's feeling unhappy with the road conditions - generally only when you try to use autosteer off the dual carriageway, which it isn't really designed for.
EDIT: Actaully, i just checked the manual - when autosteer is speed restricted, it's limited to speed limit+5mph, so that wouldn't precisely explain the behaviour you're seeing, unless it's changed between software releases...