I think the OP is being ridiculous in judging people for not knowing why Christians celebrate easter and more than a bit of a hypocrite - but I do think religion and especially Christianity should be taught in British schools.
Not from a 'this is the truth' point of view but it is important to understand the underlying fundamentals of the different flavours of Christianity because so much of our history and literature is tied to it.
Without a decent knowledge of Christianity, Shakespeare, the Canterbury Tales, Dickens, Chronicles of Narnia, even Lord of the Flies become much harder to fully comprehend.
You also don't stand a chance of a thorough understanding of history - the Crusades, pretty much the entire Tudor period, the English Civil war, the Jacobite uprising, the Troubles - religious differences played a part in all of them.
An understanding of other religions are just as important to fully appreciate more modern history as well as current world politics.