Unfortunately, T-Net's "enlarge image" option isn't working for the photos you posted, at least not with my computer. Please spell out as much of the backmark as you can. Use a dash/hyphen for letters you can't read. For example, S--VI--.
In the front-photo, your button looks like a low-convex front. Please post a "sideview" photo so we can see how much of a dome it has.
More photos of the front and back could help, too. Usually the lighting is a bit different in different shots, which can reveal details that don't show in some views.
Your button seems to have a "perfect-circle" thread-loop, which is a characteristic of British-made military buttons during the 1860s. (See an example in the photo below, a British-made Confederate Artillery "A" button.) Of course, that doesn't automatically mean your button is British-made... but it might be a helpful identifying characteristic.