In amazon.com, in spring sale today (don't know the duration) you can buy IV for 5$ with lots of other games, like L.A. Noire around same price. The price is available only in direct download versions. That is about £3?
I'm not a big fan of digitally downloading games. I'd rather have a physical copy.
I'd prefer physical games, but if I could get GTA IV for $4 instead of $20, I'd just get the digital copy. I usually just let the downloads run while at work anyways so the games would be finished installing by the time I get hime.