There is no need to look at a calculator as the way you are using one is wrong.
Instead of comparing costs over the life of the mortgage (because as ampthill pointed out… it means you’ve assumed the overpayment won;t be transferred once one is paid off), use the MSE calculator and sum the total loan value of the 2 mortgages after say a 5 year period (before the pay-off & transfer would matter).
If you assume a 25 year repayment period for each, after 5 years:
Overpaying on the smaller mortgage = Total debt of £197,990
Overpaying on larger mortgage = Total debt of £198,103 (+£113)
From this you can see that total debt is reduced more quickly by overpaying on the smaller (higher interest) mortgage.
The effects of compound interest only makes this more pronounced as if we look at the values after 10 years, the difference is even greater:
Overpay smaller = Total debt of £140,977
Overpay bigger = Total debt of £141,486 (+£509)