The genuine packaging is usually a white box with an embossed crown on, rather than a printed yellow one. They also have little cardboard boxes with tissue paper inside. If you are unsure of an items authenticity, look on the pandora official website and see if it appears on there, if not it is fake. However, one exception at the moment are the brand new charms, I haven't seen those on the website as yet. Hope this helps 
I have used this seller before, and even though it is Australian, the postage is just as quick as if it was from the UK! The items are also genuine
http://stores.ebay.co.uk/bjewelleduk

I have used this seller before, and even though it is Australian, the postage is just as quick as if it was from the UK! The items are also genuine

http://stores.ebay.co.uk/bjewelleduk