It would seem that the airlines have the data through APIS and I suppose will not do anything more than they do today. That is until they have to 'verify'.
But it is the 'verification' that isn't clear to me. Will they need to verify the APIS against the document or the data input on arrival, or both?
What happens if the data is inconsistent? Do they refuse to board? Why? They are leaving. What happens if the airline staff make a mistake and verify something that shouldn't be, or vice versa?
What happens if the passengers document changes after arrival in the UK? A replacement passport, for example?
When/where will the check and verification be done? Only logical place/time would be at boarding slowing that process.