To be fair, if Steve is referring to the Paddy Power right next to me, then I can't see anyone getting refused unless they are in there every single day winning fortunes. Not sure about the other shops but this one does seem a bit different.
I can't recall ever being refused or scaled down from the advertised price in the PP shop to which you refer. Then again I've had broadly similar experience at Ladbrokes, Hills and Coral as well as with a small North London independent who would try to accomodate you at the price you asked for rather than any particular chalked up price (...when it was still in business that is).