Unless you are married, there is no presumption that items bought during the course of a relationship are jointly owned. So ownership comes down to whether a) there is an express intention that they are shared items or b) the fact of legal ownership - i.e. who pad for the items.
A further difficulty is that second hand goods aren't worth much, and to replace them costs far more than the second hand equivalent is worth.
Disputes as to ownership are usually resolved through the small claims court, if you can't settle this amicably. I would suggest thought that it is worth attending a mediation session with an accredited family law mediator to see if there is a possibility of agreement without having to take any formal steps.
The link here allows you to find someone near to you: https://www.familymediationcouncil.org.uk/
And this link here tells you more about the mediation process:
I hope that this answers your question?
Thank you for your enquiry today. I am happy to answer follow-up questions - please do get in touch with requests for extra information or further queries and I will do my best to help you.