Owning your own home

Otherwise:

To answer your original question, if those were your only choices I'd do both. Reduce the monthly payment to £550 or so, then overpay by 10% a year which would take your monthly payments to something like £1200. This will shave a chunk of those 17 years off, penalty-free.
Then, in 2028 decide what you want to do during remortgaging - you could pay a big chunk off then.

is a succinct way of putting the "if you think there's a risk..." part. Gives you a bit of flexibility (in case life happens) as you can drop to £550/mo mortgage payments and the sums involved aren't as eye-watering. More than likely you'll have saved enough way before 2028 to pay the lot off.

Investing money to try and beat mortgage rates is still a bit risky and is time consuming and stress inducing. Any windfalls I get will be going straight to my mortgage as I know it'll be much easier that way.

IANAFA, etc.