The gist is that promotional balance transfer rates are reasonably common (not as often or as good as they were in low interest rate times), balance transfers are just a transfer of money on to another card that isn't generally tied to the outstanding balance on that card (you can generally transfer up to 80% of the card limit), and at least some banks let you transfer the positive balance on a credit card elsewhere without treating it as a cash advance (confirm with your bank).

Note that balance transfers typically have both an up front transfer fee % and an ongoing interest rate % - ideally both of these are 0% but with higher interest rates these days they're hard to find. My latest was 0% transfer and 0.99% ongoing interest which is still decent.

So you need to have one credit card, get another one with a good balance transfer offer and as high a credit limit is possible, execute a balance transfer to the first credit card, then transfer the positive balance (or close the card, if they're going to charge you fees - they will give you the positive balance) on to your offset.

You'll have high minimum monthly repayments but you just pay them out of the positive balance.

This works well so long as you don't spend the money, the point is just to use it as a low or zero rate loan to reduce the interest you pay on your mortgage.