This is the kind of thing you'd find in waitrose while it's in season, which is summer - early autumn. You'd also should be able to purchase it from a number or organic farms that ship out veg boxes. They tend to call them globe artichokes in the UK, I assume this is to seperate them from the Jerusalem artichoke.
The only place I could think of possibly finding one out of season is a posh, specialty grocery. But I'd call ahead to check if they have any imported in.
Good luck!
