If POS means "point of sale," have you looked into the free app called Square? You have to order, from the company, a little dongle that goes into the iPad's head phones slot. When you receive the dongle, you sign up with the company so that you can accept cash, credit or debit card transactions. Then, it's a simple matter of opening the app, swiping the card (or typing in the cash amount) and you are off to the races. Oh, and from what I read, Square's fees are pretty reasonable.
From wandering around the Interwebz, it appears as if the app is available and can be used in the UK. But, I'd just recommend going to the company/developer's web site and reading up on that, among other things.
Might that work for you?
Marilyn