I really don''t know the legalities, but my personal suggestion would be to take with you some personal ID and whatever documents you have that show you are co-owner (or co-tenant) of the house. Then when you come back, if he has changed the locks and refuses to give you access, you have appropriate documents to show a locksmith to get access and further new locks. Bear in mind that, to play fair, you''d then need to give STBX a key to that new lock.
If your name is on the land reg doc''s, would it not be better to pre-empt the problem by first telling her you have legal right of access and that neither of you are allowed to change the locks to prevent the other entering the property?(Prior to your business trip)
Of course you could send a solicitors letter but if you are only thinking about divorce it could inflame the situation.
My ex has done this several times. What I was advised to do by my solicitor was to keep the paperwork to hand which proves you are an owner. Inform the police of your intention to gain access to the house before entering (if upon your return she has changed the locks)Then you can ask her to let you in, if not you are entitled to get a locksmith out and obtain entry. The locksmith may ask to see documentation that you own the house. (Mine did) but you must give her a key.
The problem comes if she kicks up a stink and causes a scene. If she will physically not allow a locksmith to break in then there is unfortunately not a lot you can do. A locksmith would not risk an assault charge by touching her, or breaking in if there''s a danger to her personal safety (eg she stands behind the window he is going to break). If she starts a scene then the ultimate conclusion would be that the police are called for breach in the peace, and it is likely that you would be denied access.
Fair, no. But that is how it goes.
I would do as NoWhereToTurn suggests and pre-empt the problem by asking for an undertaking that the locks will not be changed and that neither of you will be denied reasonable access while you both are living in the property. And make sure important documents (passport etc) are kept with you or somewhere safe.