A 1985 children's fantasy novel by Pamela F. Service. This book is actually not particularly good. Certainly not her best work.
Service typically writes children's science fiction, but this leans a bit more towards fantasy. The book is set in post-apocalyptic Earth (~500 years after a nuclear holocaust). Technology in a schismatic Britain has regressed to that of the early Middle Ages. Other areas are presumably worse off, as Britain was not a popular target during the war due to their cleverly ridding themselves of all nukes before the holocaust. However, the situation differs from the first Middle Ages in some obvious ways: it's much colder, rarely thawing even in the Summer; we cut down the forests the first time around and the massive radiation and global cooling did little to help plant growth; mutants roam the countryside; Martin Luther's legacy wasn't magicked away by the nukes, so there's still not a unified Catholic Church dominating the nation, and religion does not appear to play a major role in people's lives, receiving only a passing mention. (The main character usually attended church, but he didn't attend any particular church. A different one each week, pretty much.)
The three protagonists are children aged ten, ten or eleven, and fourteen. They have an not-particularly enthralling adventure out in the wild and set up the next book (Tomorrow's Magic) for us.
Wellington Jones and Heather McKenna are a couple of losers at the Llandoylan School, one of those unpleasant boarding schools you don't want to be at. It's cold (nuclear winter), and lonely (they have no other friends), and hostile (but they have other enemies).
Heather, who likes to read and loves the historical figure Sherlock Holmes, reads in one book about buried treasure presumably nearish the school. She and Welly head out in the wild to find it. They don't, but they do find some hungry mutant wild dogs. School loner Earl Bedwas happens upon them in their moment of need and saves them from certain death. Now they're sort of friends, kind of. Except he's a loner, so only sort of friends.
Important aside: Earl was an orphan who appeared at the school some seven years ago speaking a foreign tongue no one knew. No known family, origin unknown, and very bright. Oooh! Spooky!
Anyway, Heather and Welly hear him having nightmares in his dorm room, and after that, they're even closer friends. Which is nice, because Earl can kick anybody's ass, and rich student Nigel Williams likes beating up loser kids who have no friends to protect them.
However, Earl notices some evil, evil lady in town, and she notices him. Help! Eventually, she visits the school to claim she's Earl's Aunt Maureen (and her sidekick is Uncle Garth). Earl knows this to be a load of bull, of course. He runs off and Welly and Heather come with him, much to his distress.
After a short chase by the lady and her henchman, they escape, when Earl gets a good fall off a cliff and she decides he's dead.
Surprise! He's not dead. And he's regained his memory. Turns out he's none other than Merlin, and the lady is Morgan le Fey. When trapped in the cave (you know the story, right?) he made aging work backwards and then forwards over and over again until something finally broke him free (and amnesiated him) when he was about ten years old and on the growing older stage.
This means Earl also remembers how to do magic, but magic has 'changed' over time, and he hasn't kept up with the changes like Morgan has. His spells tend to turn out purple and are oftentimes ineffective. He needs practice. He'd also like Arthur. They head off to find Avalon.
Morgan makes some efforts to stop them, and they have some other annoyances along the way. Welly and Heather grow emotionally, Welly discovering that killing people isn't all that fun. But finally they make it, and a young Arthur Pendragon (rather shocked by April
in Britain being all frosty and grey) and Merlin being a quest to reunite Britain. The end.
If you liked this...well, read the sequel.
I don't really have any other books to compare it to, but it's maybe worth reading if you're bored. It's for younger kids than most of the good stuff I read. The sequel, Tomorrow's Magic is worth reading if you liked this one, but what you really need are some better books to read, not more like this one.