For a crash course in British culture, simply talk to a greeting card retailer. It's interesting to learn which US concepts and designs translate to the UK market and which don't. For example:
celebrations

There's obviously no need for Happy Thanksgiving cards like Ess & Jae's pretty design, since the holiday doesn't exist in Britain. Likewise, in the US you'd be hard pressed to find a card that congratulates a new driver on passing their driving test, but in the UK you'd have no trouble at all [card by James Ellis Stevens].
The difference between other celebrations is a little more subtle. For example, no one really throws a bridal shower [invitation by We Do Windows]. Instead, prospective brides have their hen night, the British version of a bachelorette party. This hysterical hen night invitation by Ballistic Blue gives a clue as to the typical tone of these parties. Bachelor parties, to continue the metaphor, are known as stag nights.
The Brits don't typically throw baby showers either, which rules out the need for shower invitations. These parties are starting to pop up, however, and there are some stores that do sell bridal and baby shower invites. American expats and a growing number of English women buy theirs at Fortnum and Mason in London.
little things mean a lot
Small details such as spelling and slang can deem a design irrelevant to the UK market. Coco Press publishes cleverly illustrated journals with specific themes like cooking and art. Their travel journal, left, has the word traveler on the front [no surprise there!]. Since Brits spell the word traveller, this particular style doesn't stand a great chance. Good thing the rest of the designs translate.
Likewise, one of Remanents' greeting cards features a design whose caption reads a shore thing. This way of referring to the beach, or seaside, is uncommon here in the UK and even in the US its usage varies among different geographic regions. Personally, I'll always be a shore girl - no beach for me, thank you.
Finally, here's a cultural reference that doesn't cross the pond at all. One of Dairy's best-selling greeting cards, left, refers to Jeopardy!, a long-running American TV game show. Until it's host, Alex Trebek, starts showing up on British TV, it's safe to say that this card probably wouldn't sell very well. But give it time - no doubt Jeopardy! will cross the pond sometime soon and Alex will be everywhere!
pc? hardly!
One of the biggest differences between markets is how they treat Christmas. In the US, it's much more common to see a Season's Greetings card like this handmade one by Nikita 7 than it is to see Merry Christmas. Americans are terrified of committing a crime against humanity by sending a Merry Christmas card to someone who might - gasp! - not be Christian. Therefore, the all-inclusive Season's Greetings commands the greatest share of shelf space.
In contrast, Brits are all about Christmas, and some card distributors don't bother publishing anything else. Retailers will typically carry some Season's Greetings and Joy/Peace/Cheer cards, but nothing like the number of Christmas styles they'll offer. And don't look for any Hanukkah cards like Turquoise Creative's letterpress version because you'll most likely come up empty-handed - there simply isn't the same demand.
Happy Holidays, another big US favorite, doesn't really fly in the UK either. A stationery buyer I know suggested that this is because Holidays typically refers to both Thanksgiving and Christmas and since Brits don't celebrate Thanksgiving, the phrase doesn't resonate. Makes sense to me. You're much more likely to find a design like this one from British designer Amanda Seymour. It's nice to be in a society that doesn't find Merry Christmas an unenlightened, bigoted or anti-Semitic thing to say, and I for one hope the US pc-police don't cross the Atlantic anytime soon.