If anybody wants this, I have a detailed step by step guide to the TSA+Visa process - email me. Well checked out in 2006 and used by many, as far as I can tell.
The TSA process is easy (a website session, $130, plus a trip to Flight Safety at Farnborough to get fingerprints) but there are a few gotchas.
The Visa process is just a day wasted hanging around at the US Embassy, poking your finger into fingerprint readers on each desk and showing your mortgage statements to the officer who wants to be sure you won't end up on their social security.
Nowadays, the TSA and Visa people reportedly cross-check so if you need (and obtain) TSA you will also have to get a Visa, and this is regardless of whether US law actually requires the Visa.