Carnac, this should be the ball you saw - a Sherrin. It’s used for all high-level Aussie Rules games (with a yellow version for night games).
I don’t believe the British have an equivalent of Australian Rules Football (Aussie Rules, or in my area, just “footy”). The closest thing in the world is Gaelic Football, which is still quite different. Aussie Rules is the dominant code in 4 of the 6 states of Australia - Victoria, South Australia, Western Australia and Tasmania, whereas in NSW (especially northern NSW) and Queensland, Rugby’s the more popular game. Even so, Aussie Rules is making inroads around the country.
There are 18 players for each side on the field at any time, plus a few on a small interchange bench. The aim is to send the ball to your team’s end of the field by kicking, handballing or simply carrying it. You can’t throw the ball, you can’t run with it without regularly bouncing it (like dribbling in basketball, only less often), thus you don’t see long runs like in American football. You can tackle people with the ball, but not if they don’t have it - so the “blockers” etc employed in American football don’t get a look-in.
Your aim, once you’ve reached your end of the field, is to kick it through the larger of the four posts arranged like this:
I I I I
Kick it between the bigger two, you get a goal, which is worth 6 points. Kick it between the smaller two, hit a post, touch it with your hand before it goes through, and it’s only worth 1 point.
The game lasts 4 quarters of roughly 30 minutes each, by the time the ball is brought back in after being knocked out of bounds, kicked through the goals etc.
The referee (umpire) doesn’t wear stripes, traditionally all-white, though nowadays they sometimes wear bright neon colours. They do signal with two hands for a goal and one hand for a behind (single point) though.