You can't do this. The second charge is a charge against your parents (or one of them). The holder of that charge can refuse to allow a sale without the charge being paid off. Your parents could apply to Court for it to be sold without the agreement of the second charge holder, but I suspect the judge would want to be given a very good reason before agreeing.
If it was agreed, the sale proceeds would go first to paying off your parents mortgage with whatever is left over going to reduce the debt to the second charge holder. The remaining part of that debt would become an unsecured debt owed by your parents to the firm which had had the second charge.
You could attempt to negotiate with the second charge holder on the basis that the remaining unpaid part of the debt would become a debt owed by you rather than your parents, & would be placed as a second charge on the house once it is in your name. I would not advise this - it is never a good idea to take on someone else's debt!
As you can see, none of this is anything to do with whether you can get a mortgage. Any mortgage you get will have to be a first charge. The only possible interest a mortgage lender should have is if you agreed to make payments on the second charge debt - they would want to be certain that doing so would not adversely affect your ability to pay the mortgage.