I think this is in danger of becoming over-complicated, but Tom’s missus mentions the most important thing which is
do you need easy access to the cash?
If not, then pay off the mortgage. I’m not sure what inflation is at the moment, but most savings are barely matching inflation, so saving money seems a bit pointless if you have debts to pay off (unless you need easy access to the cash).
The compromise? Offset mortgage. I had one of these and it was great. I still had easy access savings but instead of earning interest the savings capital was offset against the mortgage, thus saving interest on my mortgage. Since interest on borrowing will always be higher than interest on saving, it seemed a no brainer to me. That’s before you even consider tax on savings interest, although that’s not relevant to cash ISAs
Obviously you have to check the offset mortgage rate is competitive with other mortgages, but I think it’s worth looking into. My offset mortgage had no over-payment penalties either, so if we had a spare lump sum we paid off the mortgage.