Just to add my two cents worth. I returned a few weeks back from completing my ppl at britannia. I did NOT bother with a visa and was not challenged by the immigration officer at all. I was worried though like you probably are as I had read earlier threads on pprune concerning the need to get a visa before I left. As it turned out I ran out of time so couldnt have gotten one in time anyway.
When I was out there a few guys at our school (and staying at the same house as me) had problems as they both put the same address on their immigration forms and when asked if they were travelling with anyone else they clearly said no as they had never met (bad luck indeedy). It all turned out okay because a simple call was made by immigration to the school who sorted out matters. The school also has a letter on immigration letterhead (I saw it!!) which categorically states that you do NOT need a visa for a ppl, irrespective of what some fine point of the law on the US embassy website says.
Mad Jock - you must have heard the same story when you were out there?