I should have been clearer on this, sorry.
A lot of people will not have received their invite letter in time for their appointment so it's not vital that you have the letter with you.
I, and many of my clients, actually received the invite letters after we had been given the jab so it would have been literally impossible to take it to the centre.
I didn't want people reading the previous post to panic if they hadn't received their letter before their appointment. As long as you take a form of I.D. with you they will be able to confirm your identity on their system when you arrive