Facebook has some great opportunities on their Grooms and riders wanted pages. The UK has a really thriving equestrian community and offers great access to top yards. The US is larger but has a phenomenal spread of knowledge. It really depends on what you’re looking for.
Try the website yardandgroom.com they have some amazing opportunities all over the world
I think it would be awesome to go to England. They have a different perspective on horses and they have different techniques . If you can, I would try England