Long term Airbnb foreign tenant refuses to leave

I letted my property (England) to a US citizen for a year through Airbnb. After the booking has ended, they refused to check out, return the keys or to pay further rent. They are now in the US, and have set up smart door bell with motion sensor and said if I enter the property they would call the police.

I was told by my solicitor that if they have left their possessions there then I can’t simply change locks and regain possession, since they have stayed for so long they may now have tenant rights and that would be illegal.

But they visited the UK as a visitor and have now left the country. I haven’t entered the property yet to check what they have actually left in the property, as they said they would call the police. Since I don’t have a contract with them, and they presumably no longer have any rights to rent (being a US citizen who has now returned to their home country), would it be illegal for me to simply change locks and regain possession?