Here are 81 pages of "guidance notes" from HMRC on how the UK will implement its IGA, and here is a story that says more will be forthcoming in the first 6 months of 2013. Remember, this is a moving target since everyone is waiting for final regs from the US. They were expected by the end of the year (i.e., in the next 11 days).
An interesting point for treaty interpretation enthusiasts is that the HMRC is issuing its guidance in the form of "regulations" for interpreting the US-UK treaty. Interesting! Interpretive regulations for tax treaties--not common, and not uncontroversial--see, e.g., here and here. FATCA continues to deliver a treasure trove of international law questions.