IMO pay off your mortgage, if as you said you will never come in to that type of money again. The only reason I can see for you to keep the cash offset is to make snap decision purchases, like cars with heated steering wheels and junk. Those you will regret. It is very easy to spend that money.
If you pay the house outright, the bank will lend against it anyway but you will have to go through the process and you are more likely to reserve that for when you come up against a real problem in life.
