They can indeed repossess when only one person hands in the keys. However, that they appear to have breached their promise/contract with you would be a very useful defence for you in court. In effect, you appear to have made arrangements for you to prevent repossession were the Halifax to have called upon you.(posted 8853 days ago)Keep what written records you have, and if asked for meony, point out that you are not getting the benefits you expected from their side of the bargain you had with them - which were ownership of the house.
Get proper legal advice as well. You need it.
Hope this helps,
Lee