Criticizing a religion and discriminating on the basis of religion are two vastly different things.

Well, discrimination according to religion is illegal in the UK, isn't it?

Also in most other western countries nowadays. In the U.S. it's been illegal since the Civil Rights Act of 1964. Historically one of the more common cases of discriminating based on religion was the prevalence of businesses with "no Jews" policies, but that was eventually banned.