It just occurred to me that trains will often arrive at their final destination well before they're due to depart as another service. You will often be able to board the train in advance too. Right now, I'm at Manchester Piccadilly station. I needed the toilet, but rather than spend 30p, I just got onto a Virgin Train that is due to depart in half an hour. The toilets on Virgin Trains flush into a tank, so you're allowed to flush in stations (not all trains are like this, so pay attention to signs that say "do not flush in stations" unless you want your business left the track in the station.
so there you have it. Just a quick tip to save you some cash.