Thanks for all that info, Tom.
We need to establish whether he served in Afghanistan or not. The only evidence you have so far is the testimony of his sister, as told later in life. Family stories - as you'll be well aware, I'm sure - must always be treated with caution, however, I would certainly give this some credence as I doubt the story would have come out of thin air (though that has happened too in the past!).
So which was his regiment? Your regimental info comes from after the Afghan conflict, by which time he could have transferred, but let's check them out first.
10th Lancers: they did serve in the Afghan War, but there is no O'Regan (or similar) on the medal roll. As this regiment is a strong candidate, thanks to the Suakim/Jumna data, the possibility must be considered that he served under an assumed name or he was missed off the roll (unlikely, but anything is possible).
M/1 RA: this battery did not serve in Afghanistan, but many men in the Royal Artillery changed batteries a fair bit. The RA batteries that marched from Kabul to Kandahar were 6/8 and 11/9, with C/2 joining at Khelat-i-Ghilzai - there is no O'Regan (or similar) on the rolls for these units.
I did check a few other RA batteries, particularly those that may have marched from Quetta to Kandahar, but could not see his name either. This is a big list, so my search was not exhaustive - more of a quick scan. He could be there somewhere.
You mentioned the York and Lancaster Regiment - a handful of men from the 65th Foot (its pre-1881 title) were in the Afghan War. Again, no O'Regan on their roll, I'm afraid.
Let's have a quick look at the other British regiments that were on Roberts' Kabul-Kandahar march, in order of likelihood -
9th Lancers: no; 60th Foot: no, there's a James Regan; 66th Foot: no; 72nd Foot: no, there's a Francis Regan; 92nd Foot: no
There are plenty of other regiments that can be checked, but without a pointer it's a big exercise. Of course they all marched over Afghanistan's hot and arid plains, so the medal roll is still an option.
I think the next thing to do is to see if you can find his service record at Kew - that should contain all the answers. Also see if there are any clues on the census - particularly the 1871 and 1891 (as he's likely to have been in India 1881).
Best for now - Garen