As @nick says the interest rate isn’t guaranteed, high now -> low later.
I’ve never seen the point of cash ISAs (from the savers perspective) particularly now interest rates are so low, how much tax are you expecting to save ? From the banks/building societies perspective they are great.
@surfer, I don’t understand how this helps you, either it’s an offset mortgage in which case you are saving interest on you mortgage (which is going to be at a rate much higher than savings rate and it’s tax free) or it’s not.
FWIW I work in finance and have an offset mortgage, all savings go to offset the mortgage.