Short lease, extend or not?

reecy_2

I feel a little silly posting this as there's most probably a very simple answer and I just can't see it
Our mortgage finishes next month (woohoo!) but the lease on our flat is down to 65 years and will cost around 25k to extend another 99 years.

My husband has spoken to a financial advisor who said that if we want to sell the flat, which we want to do next year, we would be better to leave the lease as it is and sell at a lower price to, hopefully, a cash buyer. This makes me feel a bit uneasy as it's difficult enough as it is to sell right now, and the more time that passes the more it will cost to extend if there is no buyer.

Now this is the part that makes me feel really stupid because I should know the answer, but if we were to extend the lease, would we be able to borrow that money against the flat even though the mortgage has finished? And then increase it when we want to buy a new house?

Thanks for any advice