Plus, each time you pay a lump off, then the amount of interest you pay each month will go down, so you can add a bit on to the 'repayment' bit.
So, in the example above. Let's say you are currently paying £150 pm interest. If you do nothing, and interest rates stayed the same (which seems unlikely) then you pay £150 per month, every month, and, at the end of the mortgage you have to find £35 000 to pay off what you originally borrowed.
If you save £200pm into a saving account and each year do a single payment of £2400, then the actual amount you will be paying interest on, goes down.
So, say you are paying £150pm on a loan of £35 000, next year you will be paying {wild guess} £145pm on your new amount, of £32 600, so all other things being equal, you can then save £205 pm and have £2460 to pay off next year as your lump sum. It doesn't make much difference in the first couple of year, but really does make a good difference 10 yrs down the line.
As you are saving it into a savings account, you will get a tiny amount of interest too, plus, if you have some kind of emergency, or change in circumstances, you aren't actually committed to be doing the over payments. OTOH, if you find you have extra money over what you commit to saving each month, then you can throw it in the savings account and pay down the capital quicker.