Not true. Once the tax year is complete on april 5th you can work out how much you've earnt, how, how mauch tax you've paid and then how much you should have paid. if the former is greater than the latter you will get a refund otherwise you owe the tax man.
You can only claim all of it back if your earnings so far this tax year were less than your annual tax free allowance (around �6300 i recall) and if you can show you won't earn any more. It also depends on whether you have any redundancy pay- the first �30k is tax free but anything over that is taxable.
You need to get a P50 from HMRC http://www.hmrc.gov.uk/incometax/stop-work-ref und.htm
Currently level of tax free allowance is 6475 if your earnings are under this then you can get a refund on a P50 but if you are claimimg JSA then you cannot do the form the JCP may deal with a refunfd if due or you may have to wait until the end of the tax year when you get your P60.