People can think what they want. It’s acting on unacceptable ideas that’s wrong- and that’s what we have the law for
I think most people, generally speaking, feel comfortable and safe when they’re surrounded by people with whom they have a certain amount in common. No family or community or nation can survive if every individual has differing views on the essential tenets of society. We need to agree on basic things to make the common project work.
I agree that what people think is entirely their own business. But those thoughts translate into action - on a small scale within a family unit, or into violent action as they did at the Charlie Hebdo offices, and on Westminster Bridge, or political action, eg. Brexit. It damages a society to be so fractured.
That’s why we need to make clear what our British values and principles are. If other people agree and think along the same lines, they are welcome. If not, they may decide they’d be happier elsewhere.