As of January 2018, the Women in Prison appeal for old Christmas cards has closed. However, it may reopen in the future. If unsure, email them on info@womeninprison.org.uk or tweet them on @WIP_live. (The appeal was for the front halves of old Christmas cards - the bit with the picture on - to reuse for card-making workshops.) Sources: http://www.womeninprison.org.uk/about/donate.php and https://twitter.com/WIP_live/status/950408291053592576
As of autumn 2017, the Woodland Trust/M&S Christmas card recycling scheme is no longer running.
Source: http://www.woodlandtrust.org.uk/blog/2017/10/update-on-christmas-card-recycling-scheme/