I'm surprised by Theresa May's reaction. I know she's a Christian and obviously the Christian celebration of Easter will be important to her, but there's no reason why people who aren't Christians can't celebrate Easter in a non-Christian way, e.g. by going on hunts for chocolate eggs. Oestre, from which the modern name derives, existed before Christian Easter.
Also, why shouldn't a chocolate manufacturer leave out mention of the religious part of the festival? Cadbury's hasn't set itself up as a Christian company, has it? Not mentioning a religion is not discriminatory. Christianity does not own Easter any more than it owns Christmas, even though the Church has, historically, purloined those festivals.