Sophie, the rules are very strict about sending explosives through the post, and the Bang is counted as explosive. I wasn't allowed to list Party Poppers on eBay if they could be sold outside the country, because they have a bit of explosive stuff to make them Pop.
It's the Bang, not the shape, which is banned - if you made a tubular cracker with the gift inside, and wrote "Bang!" on a bit of paper inside, I can't see that being a problem. Just don't write "crackers" on the Customs declaration.