This is a great question, and of-course we can only speculate as to what a hypothetical 12th century /u/kinder_bueno would have known about lions. So here is my speculation:
Just for the sake of ease i am going to answer this question assuming you are an English Noble, Richard III was, in name at-least, "King of England and of France, Lord of Ireland, and Prince of Chester"
Firstly, i know the words "Lion" and "Leopard" were used sort of interchangeably during this era, so that certainly alludes to a very understandable ignorance. Heraldic writer Johanne de Bado Auereo (13th/14th century) asserted that A Leopard was "borne of an adulterous union between a lioness and a pard"... you know - a Pard. lol.
This is probably why the "lions" on the Royal Arms of England look a bit off keel. Quite what an un-adulterous union between a lioness and a pard would produce, we can only speculate. I don't think medieval semi-mythological interacial feline marriages were that common.
Etymologically, at-least, it makes sense Leo (Latin for lion) + pard (?). Well, Pardos is sort of Greek for male panther, quite what Hellenistic cultures thought a panther was, again, we can only speculate; it was probably a generic term for a beast. Pan -παν- ("Greek god of the wild" and/or "all" as in the pan in pan+acea (all+cure)) + thir -θηρ- (a slangy term generally meaning beast)... so all/mighty beast. Just in case you weren't confused enough.
I'm guessing if someone shouted "panther!" in the middle ages or antiquity, it was a sufficiently accurate designation for some scary predator in the vicinity. If you're first thought in such an emergency was something like "does it have a mane or spots, who were its parents?" natural selection may have politely removed you from the gene pool. Learned and or traveled folk probably knew distinctions - People contracted to acquire exotic animals for coliseums for instance. I doubt many Englishmen held that post; however stranger things have happened, and people, English or otherwise do tend to talk about and record things they have seen, or believe they have seen, which brings me to my next point.
The clergy have historically monopolized academia in England. Universities like Oxford and Cambridge were founded in the high and late middle ages as predominantly ecclesiastical institutions were young gentlemen such as your hypothetical 12th century self could become scholars of theology. You would have invariably learnt other stuff as-well but i don't think you could have obtained a degree in felinology.
This being said, it would probably have been your best education on lions, they are mentioned in the bible alone about 100 times. Not to mention the other documents sanctioned by the church that you could read to your heart's content, provided you learnt Latin, were trusted, and wealthy of-course; this was after all pre-Luther. You might also get exposed to things like the previously linked Aberdeen Bestiary, other Bestiaries, and of-course travelers tales, all of which being of questionable veracity. You may well yourself have been a traveler, your king may have taken you on a crusade for instance where you would have been exposed to a lot of things that might surprise you.
But back to the bible! Samson killed a lion, and used its carcass as a beehive.. When you look at stories like this combined with passages like Jeremiah 4:7 and Proverbs 30:30, you might see how such talk would seduce the egos, or at the very least, the imaginations, of English knights, crusaders, and kings.
I think your impression of a Lion would have been of a quasi-mythological, foreign, and ambiguous creature with fierce virtues and four legs. With other more nuanced spiritual and abstract qualities
If you think the lion is an inappropriate and altogether esoteric choice for England's national animal, as i used to, please bare in mind our queer neighbors: Scotland has a unicorn for crying out loud, and Wales, a fricking dragon.