You don't deposit the extra funds into your mortgage, you deposit it into a current account. The funds in the current account are 'offset' against the mortgage and therefore reduces your repayment.
The more money in your current account, the more your mortgage is reduced.
All banks cannot advertise in that manner because no other bank offers that product.
Only IIB offer something similar where you can deposit money into your mortgage and withdraw it at anytime without penalty, but it is not a current account.