Lemon, that's interesting and makes sense.
Yes, I can see the national booking site allows those actively lookimg to book as they move through the ages. If there was a bit more publicity (TV ads,mbillboards etc) then more people might realise they don't have to wait for a letter and book online as their age group becomes eligible.
But yes, I can see that lots of people won't be as pro-active or keen, especially as we go down the age groups and their sense of risk to themself of the virus reduces. Some people won't give it any thought before their letter comes. Whilst some actively look at the government website daily and read these threads so they can be done ASAP, others have zero interest in it all. When they get a letter they will forget about it or lose it. Some will need a letter and several phone calls.
Someone over 75 here who has decided not to have it said they have received 8 contacts about it. I thought that was pretty impressive actually. Not sure how many times they will try to contact those in their 20s who don't take up the invitation quickly, but I gues it will be several. It all takes a lot of time and work.
People on these threads are usally keenies. Most want it done as fast as poss. However, even on these threads it emerges that quite a few aren't prepared to travel more than a couple of miles to get it and will wait. I guess the government knows that will happen too. Some will travel miles for the first jab available to them, others won't be bothered by the letter when it finally arrives and will need quite a lot of work to get them to have it and will only ever take it if it's practically bought to them on a cushion at their house.
On these threads, people find it hard to udbertsand these differing attitudes, but the government will know all of them are realities and will be planning to accommodate them.
I do think the government could do themselves a favour though and make it clearer that it's possible to book online before a letter.