Horse Riding HorseRiding Horse
Phrases - Why "riding" and not just "riding"?
It is probably just a fusion of the terms "ride" and "ride". Combine this with the fact that you generally go riding on the back of a horse, and you have a prescription for general use. It is my guess that it is still from the time when a trip with a horse could be carried out on the back of a horse or in a cart or coach behind a horse.
Like TomH said, riding is a mainly US use. It is generally not in English. It is evident in the UK that riding means riding on your back. Wherever this is discussed here in the UK, it is simply called riding. Suffice it to say, riding, because it is clear what is intended by the contexts.
There is a discrepancy between the two ways of saying it, depending on the local taste of English. Riding is used by Americans who speak English, and riding is used by non-US English speakers, English and other UKers. And what if you ride a horse upright, like a rock show or a show at a carnival?
Though the actor is on a horse, somehow I wouldn't call her "on horseback". "In this case it is definitely "Riding", but you must indicate "Standing up". "It is interesting to make a comparison between the use of UK and American Englishs. This is the NGram for the German body:
Now, look at the US body of English: The two terms were about the same in UK dialect until 20 or 30 years ago, when the "back" version quickly became fourfold more common. It seems to have been fourfold more common in USE. This may be an indicator that US language use is being brought into line with UKE.
In other words, the response to "is it British English" is: No, not until recently. It is interesting that Kevinspace has raised the question of riding while upright. American English means "bareback riding", so perhaps "horseback riding" means the distinction between riding with or without a seat. At the first horse racing, which I attended at a fun market in the 1950s, the Jockey's were not on the back of the horse, but behind the horse in Sulkys.
From the following Music Man line, after a comparison of the corruptive impact of swimming pools (compared to healthy billiards), Harold Hill in Ya Got Trouble, Hearin' him tell about horse-race gamblin'. It was a competition in which they sat directly on the horse! Guess there was a significant emotion against horse races once.
Since it was so everyday before the vehicle came, you only said "riding" and today all those who are riding on a regular basis or who have become adults do the same. This whole riding gear is just crazy - it's just RIDING!