Would the post office accept letters that have old postage stamps on them? I have many old stamps, some of them marked 1st or 2nd class but most with monetary denominations on them, from 3p up to 90p. I put some on cards at christmas but I am not sure whether they were ever delivered. OK it sounds mean but my argument is that they were bought legitimately, the PO has had the money all this time and therefore they still owe me this service. And the stamps serve no purpose sitting in my drawer all the time.
All post-decimalisation stamps remain valid indefinitely. Those with a stated monetary value (e.g. 3p) retain that value. Those marked for a specific mail service (e.g. '1st') remain valid for that service, irrespective of any price increases which may subsequently occur.
This question was only asked the other day (I was interested as I still have some very old but still current stamps) - have a look here http://www.theanswerb.../Question1007171.html