Overpayment vs Stashing the same amount into an ISA

Another question,

I was talking with a friend who told me that he was recommended by an IFA to overpay his mortgage but into an ISA that has an higher rate than his mortgage. The idea being that when it was time to remortgage, he would withdraw the savings and use it to lower the LTV ratio on the remortgage.

I have looked into this idea for my own situation, and based on the deal I am looking at with the Coventry BS @ 3.99% fixed for 5 years I can not find an ISA that pay a higher rate.

As they offer the ability to overpay 10% annually, in my case I believe it makes sense to overpay directly and sweep the remaining money at the month end into a savings account to be used at remortgage time etc...

Have I missed the point here or does that sound about right.

Thanks