This video might assist
http://myfigurecollection.net/blog/16625
He mentions all the big official Japanese sites that are safe.
Infact
http://myfigurecollection.net/ is a great reference site, items with known bootlegs are flagged on their page with examples of the fakes.
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As for UK based stuff
United Publications are good, but you need to ideally pre-order as soon as it goes up as they can't always get stock otherwise
I've used Eye on Asia and they were good for in stock stuff, another user has mentioned a pre-order that never seems to have gone into stock though
Ixudeviance -
http://www.ixudeviance.com/ - are good, I've bought 3-4 things off them now. Prices are good (higher than buying from Japan, but lower than when that parcel from Japan gets hit by customs) and they seem to have a decent range available, a backorder item was also fulfilled within a week.
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As for UK sites to avoid, the only one I've heard bad things about is Tokyotoys
For ebay watch for Chinese version or 100% new (because it should be 100% official/authentic)
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Edit
Looking at the Figuarts Zero line, those figures came out a few years ago, which in the figure world makes them tricky to pickup. You tend to have to pre-order or buy as soon as things come into stock as otherwise you end up being reduced to scanning ebay or similar.
http://ekizo.mandarake.co.jp/shop/en/ is a reliable source of 2nd hand figures
AmiAmi also offer 2nd hand stuff with a reliable grading system
If you want new, they are out there, but then it becomes a game of scanning ebay and trying to find real ones