Chris,
I've heard from several different sources of carp in the Ouse and it's tributaries, in both the tidal and non-tidal sections. I vaguely remember seeing Mick Wood with one (possibly 17lb?) in Angler's Mail from the Wharfe years, and possibly one bigger than that turning up in a match in the tidal section of the same river. Have heard of big ones getting stranded in fields after a flood, mid-20s coming from way down on the tidal Ouse and have caught one myself (a carp, not a mid-20!) from one of the non-tidal stretches I fish, though it was only about 6lbs.
I think it would be safe to say that carp are likely to be present in most rivers?
Matt