You pay a bit more for an offset but the idea is that money is available if you need it, as others have said.
You need to ask the Bank who are offering you the mortgage.
I had a terrible time with RBS with a 150k mortgage.
and paid off £100k just as you are planning to do
and asked the clerk what my monthly payments went down to
and No 1 said i dont know I dont do mortgages
At least he was telling the truth - I have respect for people who say 'there is no one here who will help you.' The statement is usually 100% true and 100% accurate.
and No 2 said: Oh it doesnt ! we go on taking £450 out of your account !
So naturally I asked what the point of the mortgage was - and she DIDN'T say oh more money for us !
and No 3 said ah, no reduction, it is just that the £450 pays off more of the capital amount than it did.
and I said: but this is an interest only mortgage
and so to her credit she said, oh I dont know then - I think you have to negotiate and remortgage.
I paid off the whole sum.
and they never sent me a how-did-we -do questionnaire - I thought because they knew how they had done.
So when you apply, make sure you ask all the right questions.