Ask an Property Solicitor. Get an Answer ASAP.
Are you asking if they are liable to pay rent?
Is the house substantially different from the one they should have been living in?
In that case they are liable.
There might well not be a formal agreement but there was clearly an agreement. They were living in your house.
It might be a point that they could take if you were trying to rely on the contract to enforce minutia but not to collect rent.
Please don't worry.
They are liable. A court would force them to pay.
Can I clarify anything for you?
It is not your problem. The property was there for them. If they chose to live elsewhere that is their choice.
You can't just terminate though. This is an AST and you need a court order to evict them.
Yes, wait until they are two months in arrears and then serve the notice on them.
You will be guaranteed a possession order.
Then you could serve notice using S8 and S21.
You will need a court order but you do have an argument.