Mortgages are not very efficient if you use it to, say, buy a car with some of it. But very efficient otherwise, as peolpe say, it’s just how you pay it back that causes the grief 😆
If the ISA money you have means you can drop into a lower band for interest then I’d say go for that option.
Out of interest have you done the sums for the new repayment mortgage based on the larger figure you need to borrow? I think ultimately that’ll decide for you. Going from interest only to repayment is a fairly big shock before the fact you’re actually getting a larger loan.
My curent mortgage is just ending but it allowed me to overpay every month, this reduced the interest but also it built up as a little ‘fund’ I could actually take out again (or use for a payment holiday etc.) if required, which was nice to know if things went wrong.
Keep enough in the ISA for rainy day money, get the mortgage as low as possible with the rest – that’s what I’d do.