Hi Torjus Gaaren,
the answer to this is massively complicated, the rules have been laid down by a number of agencies in the UK including DEFRA and HM Customs & Excise (there are others as well, but I cannot remember them all off the top of my head).
Effectively it is treated in a similair way to some food stuffs, although they may be safe and do not pose a threat (through infectious disease or contamination of some form) there is a chance that they might, so if you cannot legally ship it into an area you control the spread of whatever they are worried about.
That's about the best answer I can come up with for you (I only know this as a friend asked me to check about moving one type of food from one place in the EU to the UK and if it was possible to do a similair thing in reverse).
As far as what can be imported, most of the leather / hides that we have available to us in the UK are vegetable tanned or chrome tanned. On the other hand I have imported Brain Tanned moose hide from the States and it made it through customs.
I would say that any form of tanning would be classed as a chemical process, be it chrome, brain or vegetable tanned hide so there should not be a problem with import of these items. I cannot honestly tell you about sinew, but if you find an answer please PM me with the details.
In fact if you can let me know of anyone that sells moose hide in the EU/UK at a reasonable cost I would appreciate it.
I hope this helps with some of your questions, all the best
Leon
