So the government is proposing all sorts of things, and never U-turning ever, no no no, but basically it seems to want to track everyone coming in and out of the country.
How does this work for a person with dual nationality, who enters the UK on a UK passport, but leaves on the other passport? Without any special provision it's going to seem like that UK passport holder is still in the country, and there will be some other unbalanced record of some random foreigner leaving the country. This might raise questions of data integrity, but is more likely to be ignored as "not an issue", particularly if that passport number isn't flagged up in any way, given that we're far more concerned about illegal immigrants than illegal emigrants
Are the government somehow assuming that they will have full lists of $other_country passport numbers tied to UK passport numbers for these individuals, to correctly track them? With or without the co-operation of $other_country? Or is this just one of the things they've not thought about yet?