The one time I've been to Five Guys was in New York. The burger was alright, but it certainly didn't live up to the gushing enthusiasm my friend was giving it. I'm pretty sure it wasn't the equivalent of £11 but that was a few years ago and I'm sure they charge more in the UK. I've been to so many burger places which charge a similar price and give a much better burger.
The model is $10 burger = £10 in the UK.