In Britain, happily, there is no at-will employment. Employees have rights protected in law. It isn't legal, for example, to terminate someone's employment and make their severance package conditional on them training their replacements. But I hear stories like this coming out of the US all the time.
This doesn't stop UK employers trying to circumvent these rights any way they can, of course. It isn't helped by the weaselly words in some of the law. For example, an employee is entitled by law to 'ignore' a clause in a zero-hours contract which prohibits them from taking on work with other employers. But it is not in itself illegal to insert a clause like that into the contract in the first place. This places the burden on the employee, and makes it difficult for many autistic or otherwise vulnerable people.