Tenants won't hand back keys

Other

We had a house fire in February 22 and our tenants accepted that they caused it and moved out that evening. The house is uninhabitable due to smoke and fire damage. They left a lot of their possessions in the house. One of the five student tenants sent me an email saying," we have moved out", around the time of the fire. They did not hand back their keys.
Since then they have taken the Council to the house to try and find fault, as they wanted a rent repayment. They have been back and forth to the house to get some belongings. I'm worried what will happen if they leave the front door open or take friends over to the house and create a mess. I have asked them for the keys, but I have had no reply. Their tenancy ends on the 30/6/22.
I live four hours away from the property and can not keep an eye on things. If I change the lock on the front door, is it illegal eviction, seeing that they have stuff in there and they still have their keys?
They have not paid any rent since March 22. How can I stop them from gaining access to my house without being reported to the Council?
(The damage to the house is in excess of 100k, so keeping the deposit is not going to make much difference.)