What is up with HMRC?

I did my self assessment online the back end of January. I ticked a box that said something like:

Do you wish us to collect any underpaid tax in the next financial year via PAYE.

I walked in tonight to a big red letter dated 06 February stating I had missed a payment deadline and I owed £132.

I rang the HMRC and when I finally got to speak with a human I firstly queried the date of the letter. Apparently this is the date the letters are processed but not sent out. Very weird.
I then explained my ticking the option to have any underpaid tax collected over the next financial year. I was told that the deadline for that option was 31 December, so I queried why I was even offered the option in the first place then…

It just does that apparently…

So, millionaires are avoiding paying tax like the plague, yet I’ve GOT TO PAY IMMEDIATELY or the debt will be passed to a debt collection agency.