Sending a memo after a meeting is good policy. It makes sure that both sides fully understand each other's position and you agree what was said even if you don't agree with it.
Was the memo a fair record of the meeting? You disagreed with your employer and you said so at the meeting. Were your responses recorded?
I would suggest the following.
Be firm, but polite. Send a reply acknowledging the letter and pointing out any issues that came up at the meeting that were omitted from the letter. Again, in a polite way.
Express your disagreement and your disappointment. Note that in 6 years you have never been disciplined at all. If you have received praise or promotion note that.
If you can think of other issues which you did not raise at the meeting, bring them up as well. "On reflection, it has ocurred to me since the meeting that customers have often noted how helpful I am to them..."
Don't raise the fact that others received the same letter or that this might be linked to redundancy.
Think in terms of how this letter will look at an Employment Appeals Tribunal. You need to appear reasonable and to have a positive attitude.
If your employer provided you with any factual criticism, or with feedback on some aspect of your performance which you agree with and can improve, then you should acknowledge this. And make sure to improve it.
In the good times, most employers were too busy to do things properly. You should have had regular written performance reviews. If there was a problem, he should have brougth it to your attention. Employers might have more time now to do these things properly.
I would be very surprised if he is doing this with a view to avoiding redundancy. However, he is probably doing it to decide which staff are to be made redundant. In selecting people for redundancy, employers can use the disciplinary record as long as it was compiled fairly. If you formally disagree with his assessment and you are subsequently chosen for redundancy on this basis, then you will have a good case for challenging the redundancy on the basis of unfair selection.
Brendan