I just got some Dr Beckmann Non-Bio Travel Wash at my local Tesco for 41p. It is a 100ml tube and it says it will do up to 20 washes. I've just tried it on a new bright red t-shirt (to stop it running into the rest of my wash) and it seemed to do the trick, smells nice and fresh.

As you now seem to get less baggage allowance every year I think it could come in quite handy at 2p per wash!

I think it was marked "discontinued" and it was with the wash stain removers etc in the detergent aisle if anyone wants any.