[ralferoo]

In the UK, the rules for statutory notice period are a bit complicated. If you've been working somewhere for less than 2 years, the company has to give you at least a week's notice. If you've been working more than 2 years, it has to give you at least a week's notice for every complete year worked, up to a maximum of 12 weeks. Legally, to resign you only need to give one week's notice.

In practice, most permanent jobs with fixed hours have a notice period of 1 month, and 3 months if you're somewhat senior (i.e. not junior rather than very senior). Typically, 3 months is normal until you get very senior, and then you may (rarely) see 6 months, but instead people are normally retained by offering large bonuses that get forfeited on resignation.

If you have a contract that gives you a longer notice period than the statutory minimums (which almost everyone does), then normally the notice period is the same on both sides. If you agree this in your contract, then the longer notice an employee has to give is increased from the 1 week. I suspect this is why almost every company has a contract that specifies 1 or 3 months, because it reduces the company's risk that employees will suddenly disappear and that's more important than the cost of paying someone a bit longer if you've decided to let them go.