Offset mortgages let you link your current and savings accounts to your mortgage. The lender will offset the total balances of your linked accounts against the amount you owe on the mortgage each month, and then work out your mortgage interest on the lowered balance. You won’t earn interest on the current and savings accounts while they’re linked to the mortgage.
It doesn’t matter whether you have a lump sum to offset or not, regular deposits into your savings account can save you hundreds in interest on your mortgage.