Yes you can. If the property is held in a joint tenancy simply send a letter to your ex unilaterally severing the joint tenancy. This will have the effect of transferring your shares to a 50/50 tenancy in common, which your ex cannot oppose. Notify your local District Land Registry Office of the change on their Form RX 1, it can be printed off from the Land Registry website. There is no fee. You can then do whatever you like with your half share, it requires no one's consent.
The above assumes that there is no mortgage. If there is a mortgage, however, you can still convert to tenants in common by the method I have stated, but it will have to be done through the lender who will insist upon doing all Land Registry work (theirs and yours) and charge you a fee of around �250. If you are already tenants in common then of course this does not apply.
As far as I am aware EVERY adult who lives in the house has to sign a form when money is being raised on that house.
At least when we increased our mortgage our daughter had to sign.