I don't disagree Karen - they should have a complaint procedure for every situation, it's just a lot easier to complain (and get quick resolution) when they have agreed in advance to follow a particular procedure which is the case when an EPA has been registered. In that case, they are particularly vulnerable to the suggestion that they are discriminating against the disabled, a protected class. Unfortunately since anyone can create a general power of attorney if you use an EPA in this manner, you're no more protected from general incompetance than you would be if you were any other bank user. If a bank decides that in order to use a general power of attorney you need to jump through hoops, they can set up their procedure that way, and provided that procedure is applied evenly, and is not in itself illegal, there's not a lot you can do about it, apart from, as you say, vote with your feet.
However, as I said, I can't access the Abbey website properly, so I can't check what they say about EPAs or POAs - they seem to have decided that if you're on a mac, you're not a customer they want.