Whey protein powder. In fact, I’ve just had some. 40g powder with 500ml water (I believe 38g powder = 30g protein).
It’s cheap from myprotein.co.uk (I bought a 2.5kg bag earlier in the year), and seems to work well. The stuff I have is unflavoured and, while I’m used to the slightly milky flavour now, I find that mixed with half orange juice, half water to be an ideal way of getting protein and carbs in post ride.
Milk is good, but the key protein – casein – is a “slower” release, so better suited to overnight (i.e. have a glass of milk before bed). Whey and others are faster absorbed, so work better if consumed immediately after execrise. I understand that mixing whey with milk slows the absorption of the whey protein, so it’s best with water.
I’d be intrigued to try the latest thing – colostrum – to see what that’s like and whether it has any other benefit.