Actually as it turns out, it doesn't matter which way you go. I think part of it though is subconscious expectation... Ie, humans have an expectation on what's going to happen or what we'll see, so when you're expecting a sign to tell you to turn either left or right, to see a sign to both is disproportionately confusing.

Well there's also the small matter of watching Sky's "Inside Gatwick" only a few days previously where they'd spent an hour discussing Gatwick's poor signage, developing and delivering new ones (yellow on black this time) and then celebrating their success.

Whither Gatwick sign-lady when I needed her?!