If an application has been declined for any other reason, a guarantor does not automatically make an unsatisfactory application acceptable.
Guarantors can only sign for a mortgage in branch. It's not currently possible to apply for a mortgage with a guarantor online or over the phone.
Guarantors should get legal and financial advice before signing because they can only be released from their responsibility when the owner is in a position to cover the entire mortgage, or if the loan is re-paid. With any important changes in circumstance for either the owner or the guarantor, such as unemployment, then the lender must be notified, as this will change the basis for the loan.