They haven't been religious for a very long time. The same is true of every Protestant country in Europe. Britain hasn't been particularly religious for at least 50, maybe even 100 years. The only religious Protestant country in the world, besides dirt-poor ones or newly converted ones, is America. And even America's mainline Protestant denominations have been suffering for most of the last 100 years.
This isn't to say that the Catholic Church is as robust as it could be. But it has generally proven more durable than the many Protestant churches.