6 for £27.99 is a third party seller the other two prices are from Amazon UK. Really have to be vigilant on Amazon, it's a minefield of tat and foreign based sellers.
Also lookout now for anything sold by "Amazon EU", because that will of course attract import duty and then VAT on the whole lot including shipping. I've found it pointless to order stuff from the EU lately. Ordering from the USA is generally cheaper overall and theres usually a better selection.
Bit late replying to this as I've been away but yes, I've seen that before. Usually its on eBay where they are trying to avoid fee's, or where they are collecting taxes and so on in advance although at £15 I think it would be tax free.
If you want to order from the US, I'd suggest using a freight forwarder. There are lots of them and if you use USPS for shipping, costs will be reasonable.