I think the vaccine is (or was) in short supply you almost had to convince "them" that you really needed it.
The last time I needed one I went to a place in London, patiently explained why I needed it and was asked the question "So you don't really want the vaccination, just something saying you've had one.?"
"Er, pretty much" I replied. "OK that'll be £20". My card was stamped and off I went.