Wow, that's a shockingly ungrateful response. On the other hand, why can't the gifter figure this out on their own. It's not that complicated.
1) Call the bride/groom to verify that they in fact sent the note. Calmly ask questions to clarify the meaning of the note, such as what your "position" has to do with their marriage.
2) Assuming they did send it, and it means what you think it means, inform them that this behavior is unacceptable and ask for an apology.
3) If they are sorry then forgive, otherwise part ways with them and tell them not to bother contacting you in the future.
I would not cancel the cheque or blast this on social media. It may feel good in the moment, but would only create more drama and animosity, and it diminishes your standing. In the end £100 is a relatively cheap price to pay to rid yourself of "friends" like this.