>>Divorce has been going on since 07/16. His father has put a legal charge on a few of the properties on 08/18 (whilst divorce proceedings have been going on). Can this be challenged?
Yes, possibly. The courts do have jurisdiction to set transactions aside in divorce proceedings if they have been entered into in bad faith and not for full market value. Having said that getting the transactions set aside might well be a costly business because his parents would need to be joined to the proceedings (thereby incurring the costs of more lawyers and the costs of the other lawyers in dealing with them) and the court would probably need a separate hearing on the issue.
It depends on the figures but since you mention a property portfolio it could be that it would be simpler for the court to transfer the other properties to you and leave your husband to deal with his parents.
No matter how you look at it this type of situation is so complicated that it almost invariably needs legal assistance in order to deal with it. I would therefore be surprised if you do not have your own lawyer to whom you should address the details of these transactions.