The thing for me that lets Five Guys down is the service, which is little more than an posh McDonalds in comparison. For the money that you pay I really expect them to offer some kind of table service like everywhere else.
The thing is a Bacon Cheesburger in the UK from FG is £8.45 right where as in the US the same thing is $8.70 (about £6.50). So in the US it makes a bit more sense in that you are paying a little bit more than say a Mcdonalds or Burger King for a (IMO) far more tasty burger with the same level of service whereas over here you are paying what, three times more than a Big Mac or double that of a Bacon Double Cheese XL so that makes it look more disproportionate if that makes sense?
I suppose another difference between them all is that all of FG's burgers are cooked fresh as opposed to the others where either they are sat already waiting or they are sat in the warming trays in the back.
I understand what you are saying though and I'm not trying to defend their prices, I do think it's a little too expensive for what you get however I think if we were priced the same as the US then it would make it more reasonable.