You mentioned the money would be coming out of your mortgage? Presumably that means you have the equity. If that's the case, then a remorgage would mean your repayments stay the same, but over a longer period. Since you are already paying that, I'd say that represents minimal risk.
Two things are need to be a commercial pilot: 1, have a licence. 2, convince someone to give your a job. The second is where you have to think laterally and make sacrifices. It's a cyclical industry, with waves every 5-7 years. Right now flight instructors are being sucked up everywhere, which is a really good sign. I'd say take the gamble, you'll definitely regret it if you don't. You don't have to give up the day job until you get an offer, and you could even become an FI and teach on weekends, paying off the cost over 10-15 years with a really fun hobby. You can't really lose.