Plus any problems with them and you'll have to return them to the US, ...most likely at your own cost.
Personally if I didn't already have some Berks HFC's I'd still rather get them from Tarmac Sportz for the extra £40 odd with the added piece of mind & speedy delivery.
I'm having an issue with my HFCs and Berks have kindly agreed to replace, but they are coming from the US. Tarmac has been very helpful whilst supporting my warranty claim but the cats themselves are coming from California, so there's not much in it in terms of having to go back to the US for product support.