It may be that he doesn’t need an HMO. It is only a national requirement for Large HMOs – to qualify as large, the house should be:
“Living in a shared home – does your landlord need a licence?
Your landlord must have registered your home as an HMO with the council if:
the property is at least 3 storeys high
5 or more unrelated people live in it
there are 2 or more separate households living there.”
https://www.gov.uk/private-renting/houses-in-multiple-occupation
To go back to your original question – what do you want out of this? Revenge? Or are you actually trying to pressure him into doing something? It’s not clear. If it’s revenge you’re after, just report him – why would you want him to know it was you? If you’re trying to pressure him into something then sure, let him know (but make sure you’ve checked that he definitely needs an HMO license).
In your position, I would be tempted to respond rashly but it could end up pear-shaped. I think your best approach is a polite letter or email.
Dear X,
I do have your HDMI cable. I am very sorry, I packed it up along with my things by mistake. I have put it in the post, recorded delivery, and it should arrive with you by X date.
With regard to the rent, we have every intention of paying it on the due date.
In both cases I would appreciate it if we could keep things civil; there is no need for you to send threatening text messages to me or my girlfriend.
All the best,
xxx