It will depend entirely on your circumstances and the provisions you have in place to repay your mortgage at the end of the term, such as an endowment.
Phone them up to discuss it.
If it is anything like my recent experience with Nationwide then they will require AT LEAST 150% of positive equity in the property (i.e if you owe �100k then the property must be worth at least �250k!!) before they will even consider it and, of course, you have to have some way of paying back the outstanding capital at the end of the mortgage term.