They make money from you by having your 50K in savings to ‘play’ with

Well they used to be able to invest it, but the law changed recently and retail deposits can’t be used by the investment arms of banks. All they can do is lend that money out to other retail customers.

https://www.bankofengland.co.uk/-/media/boe/files/quarterly-bulletin/2016/ring-fencing-what-is-it-and-how-will-it-affect-banks-and-their-customers.pdf

If i take out an Offset Mortgage with them of £50,000 but have savings of £50,000 then I would not be paying them a penny in repayments each month ? I know i am not earning any interest either but how do they make money from me ?

You still have to make repayments but there are no interest charges on mortgage bit. Unless of course it was an interest only mortgage, but I don’t think they exist in offset land.

What you can do, (and what I do) is repay the mortgage from the saving account as they are roughly at parity. There’s no point in having a larger saving value than mortgage because you don’t get interest on a positive balance, but I don’t just want to pay it off because I still have a good chunk of cash available in a financial emergency.

I don’t just want to pay it off because I still have a good chunk of cash available in a financial emergency.

An ex colleague did this. He had a low interest rate on an offset (back in the early 2000s when rates were higher). He was able to use the pot as a cheap source of borrowing.

use the pot as a cheap source of borrowing

It has occurred to me, but because I’m old with a good credit score I can get pretty good rates on loans as it is, and I don’t entirely trust myself to stick to the voluntary repayments! The house needs a new kitchen however, might be a sufficiently large ‘loan’ that the interest rate difference between our mortgage and unsecured would focus my mind more.

Not sure if it’s still a viable option as I haven’t used an offset mortgage for a long time, but back when interest rates were 6% ish I used to ‘stooze’ credit cards. Several credit cards all with 2-3 year 0% offers, borrow the full amount from each and offset the mortgage with it. Several years with an interest free mortgage using someone else’s money made quite a difference.