Hi...the problem is that there are a few products that will allow you to reduce your mortgage by the 80K after 4 months. You could look at higher % no redemption products or possibly offset mortgages that will have the same effect as overpaying by depositing the lump sum in the current account with no penalty. So borrow the 160K at the outset if income allows and then make the payment when available, or if income doesn't fit you could finance by bridging for the shortfall, although higher interest, will allow you to continue with the purchase.
Regards
Ian