As I was sick of just using the iPod feature on the phone even when I was just entered the tube station(!) and seeing other people not even texting but talking on the phone in the tube!  It just makes you want to throw the phone away when you need to finalize a meeting via sms or a call. After a quick googling around I came across to these 5 useful tips,
Bad iPhone Signal Strength? Watch how you’re holding it and 5 more tips. I really did not know they put the antenna at the bottom of the iPhone, where I usually hold it as a palm device.

First tip of holding the device like some kinda dirty underwear - well that pic really made me feel like that - made the trick for me in tube stations. Although still no signal in the tube. Rest was not a problem in UK or in my case at least. Last tip; taking your phone to airplane mode and back really triggers some signal search.