Just booked my jabs.
Just posting to say regardless of what happened before and my subsequent kind of low expectations for user-friendliness, the website booking system was a true work of art in good UX design. Simple, clear, fast loading, text message confirmation arrived within seconds of hitting the final "confirm" button. First shot in a couple of weeks. Makes me very happy.
I did have my NHI number handy. To save digging through filed docs if you chuck all unimportant stuff in great big box like I do, it should be listed by your name on any prescription sticker on meds. Not sure if not having the NHI handy number slows the process, it should be able to find you through name and DOB anyway.