I am probably not up-to-date on these things but currency, notes and coins, that is withdrawn usually ceases to be acceptable in general circulation at a given date and then for some time afterwards the banks will take it in exchange for valid currency and return the withdrawn one to the issuer for an equal credit - or at least that was the case. The "post legal tender" option was never well publicised, possibly because they want to frighten people into getting rid of the things in order not to have much left to clear. Not to allow any adjustment period past the withdrawal date would be extremely harsh, not least to holders outside the country who could easily be totally oblivious to something "everybody knows" (in the UK bubble). Until recently, retail banks would confirm that after the withdrawal date they would continue (for some time) accepting old currency for deposit and/or conversion. In fact, I know someone who last April arrived in the UK with a not insignificant number of withdrawn-type £50 notes. He got them exchanged through a very UK-esque rigmarole with a 2% markdown, if I remember correctly, so withdrawn currency is certainly not instantly worthless.