JBSP Question

Hi everyone, I am currently looking to buy a house by myself. I originally looked at East London (Zones 4-5), but given my budget, I am now focusing on the Southeast of England. My dad is willing to help me via a Joint Borrower Sole Proprietor (JBSP) mortgage, but I'm unsure if he qualifies because he still has his own mortgage. He is 55 years old with 9 years and £70k remaining on his loan at a 38% LTV.

Does my dad having his own existing mortgage and being 55 years old disqualify him from acting as a joint borrower for my application? I am trying to figure out if lenders will allow this or if his own debt and age will prevent us from being accepted.