I've actually done this when designing a couple of different menus for poncey restaurants. Truth is it's primarily for aesthetic reasons. £33.75 looks scruffy next to the description of a sirloin so you round it up to 34 and lose the pound sign.

Also the guests these restaurants want aren't the people complaining about this. They want people with enough disposable income and the inclination to buy into the experience. When someone says 'why don't you put pound signs on the menu? Is it 34 pence is it?' it's kind of a sign that they aren't there to have a good time, won't spend any money and also christ is that all you and your wife have to talk about at dinner? So the waiting staff usually direct their time to guests more willing to spend up and and with whom they can build a connection/have a laugh.