Well, thanks for the feedback, folks. I must say, giving milk to houseplants doesn't strike me as a good idea, but....
Hunt, re wasted water – I live in one of the wettest parts of the country. There is no water shortage. Our main problem is getting it to drain away fast enough. When I live in Oxfordshire I was careful with water and only used water-butt water or cold tea to water plants, indoors or out. Now, it's more a case of the water needing to be careful with us.
Besides, I usually use the end of the washing-up water to wash food residue from plastic containers. Some of them I even re-use for other purposes before recycling.