With all the work on the smaller cottage, we removed quite a bit of old wire and copper pipe.  All in all, there were 3 rubble bags of copper and 2 bags of wire.  I took these to the scrap merchant today, along with a kitchen sink and a mixer tap.  Total value £117.  Much more than I thought.  Looking at the receipt, it looks like I got around £30 for each of the rubble sacks of copper and £10 for each sack of electrical wire (with the plastic casing still left on, but any plugs or junction boxes removed).