If you're getting off the shelf you can buy direct from the US, they often have some huge discounts around the US holidays. I went this route before they had UK resellers but the exchange rate has stopped them being the saving it once was.
Their off the shelf is a bit different to other brands in that they've come across just about every combination of extra kit, fuel etc on a 55k and produced custom maps for those cars. You tell them what you have/want and they email what they think is most appropriate.
I'm using a tune from Eurocharged USA that is for headers. I expect that it's not as tailored as a dyno tune but, you can specify things like what fuel you use and your other modifications (for UK/European cars you automatically have free-er flowing airboxes than USA). I personally found the difference between me having a OTS Eurocharged tune with no tailoring, to a headers tune taking into account fuel and other mods made a huge difference. It felt tame before (although it was nothing but) but now it is deadly.