Aloha Ashleigh and Welcome! :wavey:
You have several options when it comes to building your collection, several of them already have been mentioned.
First, you can make pretrades for pins that are coming out for pins that you will have access to from Disney Store UK/disneystoreuk.com. Quite a few of these pins are very desirable and people will trade for them.
Second, you can purchase pins from disneystore.com (the American site) or from Disneyland or Disney World directly through DelivEARS. If you are shopping through DelivEARS, you can purchase Open Edition pins immediately but you must wait a month before you can purchase Limited Edition pins. However, since the cost of shipping to Europe is extortionate (I know this is a fact for disneystore.com and I would imagine it's the same for DelivEARS), you might be better off trying to trade for these pins or see if you can find someone who will act as a personal shopper (cost of pins plus shipping) - and maybe you could do the same for this person with the UK exclusive pins. The one advantage of buying pins directly from Disney is that you are guaranteed to know that the pins are authentic (the only way to *know* a pin is authentic is to buy it directly from Disney); the downside is that the shipping will get to be very expensive very quickly.
Third, as was mentioned, always check the sale boards here. But as a caveat, always check a seller's feedback score, don't be afraid to ask any questions about the pins, and use common sense. If something seems too good to be true, it probably is - and don't make a purchase unless you're 100% comfortable with it.
Fourth, I don't know if you would consider a long train trip (and yes, I know how long it is) or maybe a shorter flight, but if you can make it to DLRP on occasion, you could - and should - purchase pins to trade there (especially when they're having a Pin Trading Night or Event). DLRP has beautiful pins and they should trade really well (especially those PTN pins).
I hope this helps and, again, welcome! 🙂
