"...one of the pieces of paperwork you must hand to your new employer is your final tax slip from your previous employer showing how much tax you have paid in the current year..."
Are you referring to Japan?
I've worked in two states in the USA and never been required to produce such documentation.
I've always had to do that in the UK. Your previous employer gives you a form called a P45 when you leave that has information about your tax code and your earnings for the previous year.
The new employer just needs your details to identify you correctly with HMRC.
You can give them sufficient detail without handing over your P45 or income data. I have never given a P45 to a new employer and it has never been a problem.
It's possible that your tax code will reveal some information, if you're still in the same financial year, but this is an unreliable way of calculating salary.
Correct. A difference for the UK is that salary bands are often (not always) disclosed in an advertisement or told to a head hunter in advance. Not always, but enough that not doing so raises eyebrows and concerns about corporate ethics for me (and my friends).
The obvious caveat in this is that they could only estimate an average salary. You could state that your salary progresed rapidly during your previous employment, although they may not buy it.
Are you referring to Japan?
I've worked in two states in the USA and never been required to produce such documentation.