The Ordnance Survey is currently running a map return scheme, in which customers send in their old maps in return for a voucher up to £15. It runs until March 20, but they’ve already received 9,000 maps so far. (A similar scheme in 2014 yielded 10,000 maps in total.) Some of the maps returned date back to the early 1900s. (I hope the OS makes sure they’ve got a complete set of everything; it wouldn’t do to give away the the last copy of an obscure older edition for an art project. If nothing else there’s an opportunity for a crowdsourced archive here.)