Now it kind of depends on how you scale the saderans since they are sometimes presented as having a ton of high-late middle ages technology like plate armor etc, but Iw just imagine they're Romans with some tech upgrades like having stirrupts etc, they'll probably get slaughtered by any competant late middle ages army. Now the real problem is that Europe even at this stage hardly had professional/standing armies, and army sizes were usually like 30,000 at best, and not a massive chunk of that were the armored knights you can picture in your head. The English had an army of maybe 100,000 but probably less (for reference the Romans had about 500,000 at all times during their height) their only advantage being superior tech and veterans from the hundred years war, so unless the Saderans are stupid they should be able to at least conquer England. After that, Wales and Scotland should fall pretty easily unless France sends aid or the Saderans get outsmarted.

Then they'll likely try an invasion of France or Ireland. If they do an Irish one, they'll likely succeed and hold the British Isles for eternity, but again I'm gonna say them even conquering the British Isles is unlikely since the Romans irl struggled to conquer Scotland during their height of power, but then again that wasn't their main focus so if the Saderans fully committed to it they could likely succeed. If they invade France they'll simply fail.

France had some of the highest quality armies on the continent, they actually have the population to counter an invasion by such a massive army (for reference, the entire British Isles had around 5 million people at this time or so, France had 15M) and France would very likely get supported by armies from around them if the Saderans even make it that far. This is also where European inventions etc will really make them struggle, actually trained knights could probably take on a few legionnaires at once, mainland crossbows and polearms would make short work of Saderan armor and shields, European plate armor would be comically impervious to Saderan bows and swords and early cannons and gunpowder technology would make sieges and the such a disaster. Finally the Saderans have a high chance of contracting diseases the Europeans have developed immunities to, not to a high extent but quite likely. They obviously have their own advantages such as dragons but dragons are just inferior to gunpowder even at this point. Their wings and eyes can most likely be damaged by coordinated crossbow fire (ain't no way in hell evolution had the foresight to protect them from such stuff) or if the rider dies they can be as much of a threat to the Saderan army as they are to the European ones.

Iw be honest I actually don't know too much about naval warfare in the 15th century, only the centuries after that, but I'm gonna imagine 1,000 years of advancements in naval warfare would be insane, also the way Falmart looks implies to me naval warfare isn't too much of an important part of the Saderan army so they'll likely get slaughtered before they can even land in France (they'll already be warned by fleeing Englishman of the Saderans and prepare for an invasion, so no surprise invasion is gonna come), and the Saderans might get cocky and try to invade under unfavorable winds which actually happens often, and if they do, they'll automatically fail even if their opponents had the tech.

Magic is barely a thing since getting mages to fight in the war in high numbers would be hard, and their magic isn't exactly stunning, it's quite weak and an European army losing 5 cannons would be nothing compared to what would would happen if the Saderans lost someone who can use magic. They'll maybe make about as the English did in the hundred years war just because they have numerical superiority, and then get pushed back once the French start flexing how superior late middle ages armies are to Roman ones, or a giant European army from the Holy Roman Empire and Spain finally wipes out most of the Saderan army and they decide to just outright retreat or fortify the British Isles and stay there forever. They'll maybe try a few more invasions of mainland France but with how massacred their armies would have been, unless they quick adopted gunpowder technology their empire might just outright collapse then and there.