There isn't often talk of UK aquisition, so I'll answer as best I can:
The list of (known) watched chemicals in the UK is so small it's laughable. Anything in 'Category III' of the above Sourcerer link can be ordered no problem, except for possibly potassium permanganate which has recently been moved up a category or two. This can apparently be found OTC anyway so shouldn't be a problem.
The chemicals listed by NCIS can also be ordered, although more caution is needed here than for the schedule III precursors above. For a cautious bee,
any drug you can think of can easily be made without using any of these watched/listed chemicals; the hardest to do so are MDMA and LSD, but MDMA at least is still relatively straightforward to synthesise bypassing all known watched chemicals.
The list given above from Sourcerer is quite old, and new chemicals may have been added since it was made; I know benzene is supposed to be watched, and so is diethyl ether. Keep in mind also that the only people who know all the listed chemicals are the ones who write the lists - there could quite easily be an unpublished list of watched chemicals which could be used to catch people like us, having a false sense of security and laughing at the pathetic list we see.
With regards to sassafras, from the above link again:
The requirement to report suspicious enquiries shall extend to any transaction, regardless of whether the goods are intended for export or domestic sale, and shall also cover mixtures and compounds which contain scheduled substances which can be readily extracted for the illicit manufacture of drugs or chemical weapons.
I have never tried to buy sassafras, from the UK or otherwise, but have spent quite a while searching for suppliers just in case I ever have the need for any. Safrole can easily be 'extracted' from sassafras, so this may make it a controlled substance in it's own right. I am yet to read a definitive post here or anywhere else about whether sassafras is considered a schedue 1 precursor in the EU, and I suppose the best way by far to find out is for a bee who isn't up to anything naughty try and buy some. Another way could be to e-mail someone at
http://www.idmu.co.uk/
, or if they don't know, the Government backed
http://www.drugscope.org.uk
. Just don't mention the Hive, safrole, or MDMA if you choose this route!
The most important thing in aquisition is seeming professional and confident. Nowadays everything can be done by e-mail and websites, so get a company e-mail address, and start placing orders. Just be sensible. If you have to be told how to be sensible I suggest clandestine knitting instead (don't knock it until you try it).
Hope this helps