Owning your own home

What is your current tie-in period for the repayment penalty? Try a comparision tool as well as a calculator.

Might be better off just absorbing the repayment penalty and getting a brand new mortgage at the best available terms that also includes the additional borrowing I want. Is that a thing? As in, can I negotiate a cash amount to be provided on taking out the mortgage?

It is. Generally the mortgage company doesn't care that much about the previous mortgage so long as you have the property value to cover it.

I remortgaged a few years back and added extra cash for a loft conversion. So long as I had suitable LTV it was fine.

Might be better off just absorbing the repayment penalty and getting a brand new mortgage at the best available terms that also includes the additional borrowing I want. Is that a thing? As in, can I negotiate a cash amount to be provided on taking out the mortgage?

You can also often increase your mortgage (/take out a 2nd one with the same provider) too rather than ending yours early.

Other half did this yesterday as she's doing a kitchen extension and is only about 3 years into her initial 5yr fixed.
So she'll end up with, say, a 22yr 400k mortgage with 2yrs on the fixed term left
And a second, 25yr 50k mortgage with 5yr fixed

So two payments with slightly different interest rates, both from same provider.
At some point in the future she may consolidate them if it makes sense to.

Edit: Seems as though my current lender's terms are up to 35 years, so I can use the calculator to figure out my rate tolerance.

How old are you? This will determine how long they will give you.