I would have read this at some point before 2001, which is not particularly helpful but does at least cut out the last 13 years, I guess?
(Assume, by the way, that basically all of what follows will spoil the story.)
The protagonists of the story are a pair of young siblings, a boy and a girl. The brother is able to work weather magic, and close to the beginning of the story I'm pretty sure they come across a man who is being pressed to death as a witch. I'm pretty sure they rescue him.
Over the course of the story, it emerges that the setting is a Britain in which people have returned to the land. It isn't a Britain that never developed, however; people seem simply to have forgotten the use of technology. This is not a worldwide effect, however: there's a mention that, with all these weatherworkers in Britain, the weather in France has taken a turn for the dreary, rainy, and gray, and so this tech-less bubble basically covers just Britain. I think we find this out from the man, who will have come back in (and whose technological knowledge may have gotten him in trouble, though as far as I recall devices also stop working once they come in contact with the bubble). We also find out that at the moment this happened, there was an hour or two of chaos as (for instance) drivers on expressways lost control of their cars, airplanes failed, etc.
Eventually, they all come across the source of the disturbance, which ends up being the tomb of Merlin, who was apparently sleeping and then reawakened (possibly by archaeologists?) Ultimately, Merlin is responsible for Britain turning into a gigantic Ren Faire*, and after our protagonists talk to him he agrees that there isn't a crisis, that Britain should be allowed to rejoin the world, and everyone sort of wakes back up into understanding modern technology. Maybe Merlin goes back to sleep.
At the end, it's raining, and the sister asks her brother to make it stop, but he can't.
*Yes, I know this is anachronistic. Sorry.