ex roommate went back on verbal agreement to repay my half of holding deposit and then blocked me (england)

I was supposed to live with a friend of mine but unfortunately due to visa reasons I could no longer do so. I was supportive in that I gave them ample warning time and did not spring the news onto them at an inconvenient time (ie before we signed the contract). They found a replacement roommate immediately and gave me verbal assurance that they would pay me back my half of the holding deposit. For context, I paid the entire holding deposit of £400 in full as they did not have access to their finances and it was a tight race to secure the apartment. They paid me their half later and once it was clear i had to leave the country, gave me verbal assurance I would get my half back. Well, after moving back to my home country, I messaged them asking for the money, yet they switched up on me and claimed we never had this discussion and that it is a loss I should make peace with because that is the nature of holding deposits and that I should take this on with the agency. Then they blocked me on all communication platforms before I could respond.

Thing is that irrespective of the risky nature of a holding deposit, they had provided a verbal assurance they would repay me the holding deposit half- especially since I did all the paperwork in ensuring that the apartment was secured. Truthfully I do not care about the money but since they decided to throw away out friendship over this I do want to know, is there anything I can do? Litigation is not an option but they've not even allowed for discussion or negotiation. Obviously it is not the apartment agency's issue to deal with either but is there anything I can ask of them so that perhaps my half of the deposit can be returned and they can charge the new tenant? Their policy states it is up to the discretion of their agents to return any holding deposits.

Please help. I don't them to get away without even the irritation of a fight. Blocking me was so low.