The first & most important thing to do is to get all the doors giving access to the house locked. If his father can't be bothered to do it, your partner should. The bailiffs can be denied entry when they first come, but they can come in through an open or unlocked door or window even if they have been told not to.
Your partner's possessions are at risk if they get in. They will very likely refuse to accept his statements that the goods belong to him & not his father unless he can provide proof of ownership. There may be a way round this by him doing a statutory declaration but he needs advice on this. There is a website - I think it is www. bailiffsonline.co.uk - which may be helpful.
May be easier to move your partner's stuff to your place?