Yes that would be useful data. But I consider myself fairly well plugged in, and am planning a big holiday soon so am pretty switched on to travel issues. If I missed the information, I don't think there can have been much if an advertising campaign, because I would have been aware of it.

One way to approach this kind of thing, besides having a public information campaign, is to enact it over a long period of time - at least a year - so that all those people who do routinely travel back on foreign passports have a chance to go on a summer holiday, come back and be informed at the border, "next year you will need a British passport." This isn't rocket science - it should be possible to make the number of people caught out very small indeed. (Given that we can guess at least half of dual national residents have and travel with a British passport anyway!)