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it's blanket messaging done by a service
Yes, it's pretty much all automated. Hence the mid-30s guy who got notified early as his BMI was over the corresponding threshold (40).
If a human had looked at his records and made a decision they would have spotted that it was unlikely he was 6.1" tall, but the computer just calculated the BMI as 28,000, found that to be over 40 and then added him to the list of people to notify early.
It could easily be as simple as: White: Age > 40, Non-White: Age > 32, and your phone goes ping.
Different areas have completely different age profiles too, which further complicates capacity planning.
i think if your GP sent you a link you should prob go do it. They're probably taking other stuff into account, either something on your medical record, or they know they have an abundance of vaccines in that area and they want to get them out the door.
(assuming you actually want it)