Former employer demanding return of assets 18 months after leaving (they ignored prior requests to collect) - what do I do?

I left an employer in late 2021 in a role where I had transitioned from office to working remote fulltime due to the permanent closure of our local office. As part of my role, I had a chair + usual IT equipment (laptops, dock, m&kb, monitors etc) which I had carried home from the office with me before the site was closed.

I left on some very difficult terms and had an argument with my manager in my last few weeks because I refused to disclose where I was going (a client for whom I had some compromising knowledge about my employer including dishonesty, cutting corners + bad internal process etc which I did later use to benefit my new employer) and on my very last day I was verbally told I was dismissed for gross misconduct because of my "attitude" and ordered to finish early so I didnt get any formal exit interview and no further contact.

I tried to arrange collection of my assets (laptops + monitors) but heard nothing back. After 3 months of radio silence / zero acknowledgement, for the sake of covering my own back I made the decision to thoroughly format my company laptop to remove the "security risk" of having employer/client data on it and kept it in storage. I have been through a house move since then and some of the equipment had to be given away / disposed of due to issues with space etc (and in case of one of the monitors the panel failed).

I have had an email from my employer (with a pdf that looks like a letter to my old address) demanding I return all company assets - laptop, chair, monitor, keyboard, mouse etc. Looks fairly generic but they are saying I violated company policy following my "dismissal" and failed to return assets and that they want it back immediately as I am violating security policy and disregarding . This would be fine but the letter makes no reference of collection and seems to expect me to pay for it myself. Also as said some of the items I had are no longer available for return as been through a house move since then.

Pre-answered questions from/for me:

• What was the company policy on returning property? I don't remember the exact wording off the top of my head but it was fairly vague and seemed to vary depending on where you went. Something to the effect of "company assets must be returned upon exit" - my contract was never updated after we went from being office based to WFH. I did recall seeing an automated email (which I have saved somewhere) that said the company weren't interested in collecting keyboards + mice.
  Also when I left there was no official process that I was aware of. It was all flaky to be fair.
• Why didn't I post it back myself? I didn't see it as my responsibility to spend my own money to send stuff back. Also, the company never gave us a defined asset return process. It was very shaky - I had no idea which site I'd send it to etc or if anyone would receive it. If I shipped it off and it ended up in a random corner I'd be pretty buggered.
• What steps did I take to attempt return? I waited to hear something but 2-3 weeks later I got nothing so I dropped an email into a company inbox to request a return. 1 month or so in I asked an ex colleague who said they'd ask but never heard back on that front and I didn't want to bother them as they had their own problems and as I said I was not on great terms with my leadership and wasnt comfortable speaking to them again.
  I left several emails to a number of company inboxes (including one that gave an automated response that said something to the effect of "Hi, your query will be managed within X SLA" and never got an answer.
  At the 6 month mark I left them a final email that said "I will give [EMPLOYER] one week to confirm they will arrange collection of my assets. If I do not receive a response or acknowledgement within one week I will accept that as [EMPLOYER] relinquishing these assets to me for disposal at which point I will make no further efforts to contact."

Questions from me

• Was my dismissal "legit?" I actually spoke with my manager at my new employer (client of my old) and they said not to worry about it as they knew my old employer were sht, that they had not been made aware of any bad reference and that literally the only thing they got from my employer in the first place (prior to my exit / dismissal date) was a reference saying how long I worked for the company, nothing about my good performance reviews which I had on record.
• Am I liable for these assets? As said before, I gave the company several opportunities to collect equipment and emailed them numerous times even getting an automated reply from a supposedly monitored inbox. I did state that if I didn't get a response I would be taking that as the company ceding the assets for me to dispose of. I do still have some assets but some things I had to get rid of for reasons I've gone into already. It's been nearly 18 months since I left so theyve had long enough to make a call.
• How am I best to respond? I don't have too much of an issue returning things if the company collects it though admittedly there is a part I feel I am owed for unpaid overtime, annual leave I wasn't allowed to take etc but I am not paying for it. In the case of most of these assets I dont have the original boxes anymore and others would be expensive to ship.

(England)