3 links have been added on this idea.
  1. Not sure if the Royal Mail would bother with the hassle of charging more for a standard priced stamp, but it would work if it was allowed to. A stamp could look something like the one here.

  2. More info on donating normal postage stamps here. https://www.thisismoney.co.uk/money/news/article-7526527/Stick-Stamps-want-raise-cash-charity.html

  3. Good idea. there is also this way charities can cash in on stamps – I didn’t realise it was done by weight though: “Stamps are sold by weight, meaning that many charities can turn old ones into funds to help their ongoing work. Simply cut the stamp off the envelope, making sure to leave approximately 1cm of envelope bordering the stamp, and post to the correct address.”