The worst thing about taking away the UK sellers, is maybe that there was more selection. I notice it's not such a huge price difference for what I could find to compare.
eg. a DA 9150 Di2 groupset sells respectively for $2150, $2200, or $2305 at PBK, Merlin, and Competitive Cyclist... so $100 more in the US or roughly a bit less than 5%.

Ultegra 8000 is about $700 if pieced together at Jenson, but only $632 at Merlin, so that's now closer to a 10% difference.
I haven't compared wheelsets.

However, neither Comp Cyclist, Jenson or Excel seem to eg. sell 105 series groups or much in the way of the individual parts. Anyone have insight into why? Tiagra as well.. no dice

If Shimano's concern is MAP, why not allow the UK sellers to sell into US but follow US MAP pricing guidelines? At least this would bring back the alternatives from a selection standpoint.