Assuming its import duty we're talking about here, you won't be able to find this information online. It's crazy, but the actual tariffs for import duty (basically a huge list of goods and the import taxes required for them) are only available from the revenue if you subscribe to them, which varies between £300-2000 depending on what format you want.
Which is a bit of a joke, because really these tariffs should be accessible online for all!
All right then, why is there no import duty (which is charged at the same rate as VAT) on a Raspberry PI when there is on a stick of RAM? Genuine question: I'm curious to know how the rules define one item as separate from the other.
Edit: OK sorry my comment was a load of crap, I was confused. They charge both VAT @%20 and duty @~3% but duty is only charged on a massive and seemingly random list of things.
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.
EDIT: added 'assembled'. Does anyone have a link to the IR rules about this?