[mattmanser]

It's a small downside to the massive upside that in the UK most people don't have to do their own taxes, their employers and the government do it all for them.

Hence the employers need to know how much you've earnt and how much PAYE tax you've paid so far this tax year (via a form called a P45).

I guess you could get round it by claiming you had unpaid leave, or you'd recently been given a pay rise, or you'd been offered the figure by your previous company to stay.