When two people both have an interest in a property, they can either hold a 'joint tenancy' or they can be 'tenants in common'.
If you and your husband were tenants in common, you would both hold distinct shares in the house (normally, but not not necessarily, 50% each). In this case, it would be possible for one of you to sell your share to the other partner.
However, the more usual arrangement is that married couples are joint tenants. In this case, neither of you own a distinct share in the house. For example, if one of you was to die, you couldn't leave your half of the house to someone in your will. It would simply pass automatically to the surviving partner.
I think that your solicitor is saying that your marriage effectively locks you into the joint tenancy so, although you can make a payment to your husband in anticipation of a future change in the deeds (and to give you the immediate sole right to reside in the property), you won't be able to change to a tenancy in common (at which point you can officially 'buy out' your husband's share in the house) until your marriage is dissolved.
Chris