I agree 6 months is the standard, pretty much industry wide. Nevertheless, the point stands that the 6 months is only a minor legal milestone for security of tenure. Once the probationary period has passed, you are 'permanent' in the sense that you have to do something negative, rather than fail to do something positive, in order to be ousted. In other words, your employer can no longer excersise their right to give one month's notice on a mere whim.In practice, an employer who has such a whim will incentivise you to sign away your rights under UK employment law. i.e. such rights are 'compromisable' (a type of agreement which binds the parties to denying that agreement's very existence).So no real inalienable rights, and no real security of tenure.