I had a recent awful experience trying to get
ZenCart working properly on all levels for a UK shop. I would suggest that you avoid it for those uses. Got so I was wondering what the problem du jour was gonna be.
Switched to
CubeCart instead, and it was customized and running for UK within around 15 minutes. I could also skin CubeCart with a nice clean CSS/XHTML design which I had pretty much total control over. The client found the CubeCart interface much easier to work with as well.
I understand CubeCart as being free with linkback and only forum support, and pay to remove it and get active developer support. Forum support is okay, though sometimes a bit slow.