They are probably classifying their product as a 'palm-top computer', which attracts no import duty. But the component parts (the chips, etc) almost certainly will. For reasons I gave in my reply above, I couldn't tell you the exact duty amounts, but there is a link here that details the duty for palm-top computers:
That is the most bizarre list of random tax rates. I always seem to get charged a lot so will appeal next time based on whether my player is mp3 or mp4 or whatever. And I might complain to my MP about the taxes on 35mm film cameras. Bizarre.
http://customs.hmrc.gov.uk/channelsPortalWebApp/channelsPort...
I seem to remember this being the case when I imported some Beagleboards into the UK for R&D work, and didn't pay import duty on them.