I’ve seen you tell this story somewhere else – possibly the landlordzone forums or similar. You were put right on what you had done wrong in short order by some of the more knowledgeable people on that forum, as I recall. Assuming I’m remembering correctly, you didn’t serve notice or bring possession proceedings. You just entered the property and changed the locks on the assumption that the tenant had left, without the tenant having told you they had gone. You then wanted to dispose of the tenant’s property.

The police absolutely should not have insisted that the property was passed to the neighbour without express authority, but, and let us be entirely clear here, you had no right to dispose of the tenant’s belongings and, if I recall correctly, this was an unlawful eviction. Tessa Shepperson has some helpful guides for landlords on their basic legal responsibilities. Try this one on left belongings

The police were called by a neighbouring busybody and loudly insisted that I could not remove the property. They threatened to arrest me under the Theft Act and then insisted that I pass the furniture over to the busybody although he showed no evidence of ex-tenant giving authority.

I complained to South Wales Police (who are generally unfit for purpose) that they were badly training their officers with ignorance and claimed damages. So far not resolved.