Halifax's website says "If the total amount you overpay during the year exceeds 10%, we'll only charge you an early repayment charge on the proportion you overpay above 10%."
I've just redeemed my mortgage with Halifax, they charged me the early repayment amount on the full balance. (It didn't occur to me about there potentially being a permitted element of 10% even though I was permitted to overpay by that amount.)
Halifax's website says "If the total amount you overpay during the year exceeds 10%, we'll only charge you an early repayment charge on the proportion you overpay above 10%."