If you think your button may be from a British naval uniform but my Naval Gallery hasn’t helped you to identify it, this website by the Royal Museums Greenwich is very interesting:
https://www.rmg.co.uk/collections/objects/search/Button
This site has a great many excellent photos of British naval buttons:
https://www.colchestertreasurehunting.co.uk/navy%20buttons.htm
This pdf, showing the fronts and backs of naval buttons, was clearly prepared in connection with an auction so it may have vanished before you read this: if that’s the case, please let me know and I’ll remove the link:
https://www.jagermedals.com/_pdf_files/militaria_pdfs/j3627_large.pdf
Although apparently dedicated to Malta, this site has some very clear information on British naval buttons:
https://www.melitomnes.org/05-3-2-2-wwii-rn-officer-buttons-fl-large.php
If you find a website that helps you to identify a button, please send me the link and I’ll include it here to help other people.
If you can’t find what you are looking for please contact me.