I wondered that. Is it a case of getting access to the patient database, with patient records going back years, and do a new research project to find out what's happened since?
As PPs suggest, privacy advisors are very wary of data linkage of healthcare records. It's plausible that some of the trial participants or former patients are now pursuing a mix of private and NHS care for which the former is not necessarily recorded in their electronic healthcare record nor available for sharing.
The ethics of this would be very difficult. It would be necessary to see the agreements that people signed for the sharing of their data and whether they consented to follow-up and re-contact. Not least of which, given the numbers who are already lost to follow-up in that very delayed study, I wouldn't be sanguine about consent or tracking where all of these children (now adults) are now (presuming that they switched GP practices to go away to study or for work).
I would need to know a lot more about any legal changes that Javid is proposing and which data are being sort with what level of anonymisation.