Disclaimer: The description below represents my perspective. Nothing — not even what a NYC Clerk tells you — will substitute real legal advice from your local marriage bureau.
Canadian couples are safe. To date, I have yet to encounter any debate about whether Canadian jurisdictions will recognize regular NY wedding certificate. Debate about whether British, Irish, Scottish, and Australian couples need the extended (or even apostille) certificate exists….mostly in online forums.
Firstly, the apostille, extended, and regular certificates are different types. Unless they move to a country, such as Germany, Austria, or Russia, that does mandate the apostille, which is another layer of bureaucracy in addition to the extended certificate, the apostille is not necessary. To the best of my knowledge — and I officiate lots and lots of weddings for UK couples — those with ties to the British Empire do well with the regular certificate. Swedish couples also seem fine. Right now, I can only think of two circumstances in which one of my British couples needed the extended certificate immediately, and they had specific reasons (i.e. moving/traveling and a quick, surname change turnaround). Thus, following the normal path in which I file the completed license, and the Clerk mails the certificate is most likely perfectly fine. This is what the vast majority of my British couples do, and it seems to meet their legal needs.
However, I am not familiar with individual registrar offices in the UK. Perhaps different offices react differently? Maybe contacting a local office would be best? If travel plans preclude an extra visit to the Clerk’s office and you do discover that you need the extended certificate, do not panic. Once you receive the regular marriage license in the mail, which usually takes about one month, you can order the extra paperwork remotely by post.
On another hand, for the purpose of peace of mind, you may wish to return the completed license to the Clerk’s office and secure all possible documents. In this case, the Clerk will immediately process the license and give you the regular regular certificate and, upon request, the extended certificate. While I personally would not bother, UK couples could also take the extra steps for the apostille. The apostille is a “must ” for German, Austrian, and probably Russian couples.
Should you decide to return the license yourselves for the regular, extended or apostille, you must visit the office on the next business day after the marriage ceremony. Should you wait any more than five business days, the Clerk will contact me, and I will be in serious trouble: please spare me! Anyway, if you wish, the Clerk will direct you to the office where you may obtain the extra bureaucratic “stamps” for the apostille. While I am not fully convinced that this is mandatory for all international couples, I respect the virtues of peace of mind. Erring on the side of caution is route to more security and less anxiety.
Rest assured that the license process remains generally identical for residents and non-residents alike. The only differences? U.S. Citizens may use their driver’s license for the purpose of identification. International couples typically use their passports.
I hope this longer explanation helps! I know it is rather wordy.