I'm pretty sure no bank will offer you a mortgage based on what you have said.
You are effectively looking to borrow 100% of the property value which is no longer possible.
The bank will look for confirmation of the source of your funds and will quickly realize that it is coming from a loan in your partners name.
It would also be madness for your partner (I'm assuming unmarried) to give you 20% of the property value if they are not legally an owner of the property.
If your situation is a bit non-standard then you should use a mortgage broker who can help guide you towards the right lender for your situation