New DEC museum entry :D

I have a slightly OT question about the UK RCDs. Here in the US, GFIs are commonly installed on branch circuits, often incorporated in the first receptacle of a string. I don't believe I've ever seen a GFI as part of the main breaker. But then, perhaps there's a practical reason for that as our home distribution is 120-0- 120 volt, so perhaps the need to detect an imbalance on either side of the relative to neutral as well as either side relative to the other (for 240v appliances) would make the device overly complex. But here's my question: Given that the RCD is installed as part of the main breaker, surely some allowance must be made for small amounts of leakage from a number of powered devices. So how large must the leakage current to earth/ground be before the breaker trips? And a question for the US experts. On a 120v circuit, will a $6.00 GFI detect a leak between neutral and ground? Or is the detection circuitry only on the high side of the line? I hope I was somewhat clearer than mud. Cheers, Chuck