Well, Richard, despite your nice words about my knowledge of early superheroes, you don't really need me as you have listed a few tasty ones yourself. And you've added to my knowledge with Batsowl. Ta!
Just to set up some reference points for the rest of you and for myself, we're really talking about characters/heroes who have an alter ego or, at least, hide their true i/d from the world; wear some sort of costume or clothing which stands out or helps camouflage them; often a mask or hood; and sometimes have gadgets, weapons or powers which set them apart and enable them to right wrongs. So, Robin Hood doesn't quite fit the bill. His clothing is not out of the ordinary for the period and he's well known to be Robin of Locksley. Same with William Tell, Marco del Monte and some others.
Same with the great detectives.
But, to add to Richard's suggestions, There are a few other heroes/MMM's to mention. The Boy's Friend featured in 1927, a new serial titled Flying Justice, which featured the adventures of wrongly accused Roger Falcon, who is given a flying suit and has a tight, black costume , a bat's head mask, and clawed shoes.
I think we've mentioned the next bloke previously on the forum. Thurston Kyle is The Night Hawk, and although he does not keep his i/d secret, he has a huge pair of mechanical wings, headgear, goggles and a tight, black costume. He uses a revolver and grenades. This series started in Nelson Lee Library in the early'30's.
We also have the next hero on this site, in The Ranger story paper. Black Whip appeared in 1931. Have a look for yourself. Costume, mask, swinging down from up there. It all reeks of superhero/MMM.
In 1933, The Rover featured The Flaming Avenger, a genuine precursor of Iron Man. Again, you can have a read for yourselves as it's on site.
That last one neatly segues into, The Iron Man, a MMM who featured in a Newnes British "pulp" during WW1. This bloke is Dick Murray who dons a light, chainmail costume, domino mask and, armed with a revolver, does battle with gangsters, oriental villains and, at one point, a giant snake.
There is also The Human Bat, a book by Edward Home-Gall but I don't know when that appeared. Can any of you help?
By the way, Spring Heel Jack spawned a later superhero version in British comics when Spring Heeled Jackson burst onto the scene in 1977. His adventures were in Hotspur and Hornet. Set in Victorian times, this Jack(son) is a mild mannered clerk in a London police station, who dons his striking costume and power gauntlets and boots which let him leap prodigious distances. A really nice looking comic strip. If anyone would like a sample, please let me know.