Have you considered the advanced course run by ROSPA.
I believe it to be superior because regular re tests are required for the highest standards (equiv to what used to be called Class 1 driving) and the organisation is not profit making, whereas the IAM course is.
I did both in the past and can tell you the ROSPA one is more challenging. It demands a good knowledge of Roadcraft the police drivers manual and consitent use of its' procedures; something you NEVER forget.
It's debated by some (not me!) but the standard is streets ahead of the IAM.
Whatever you do, good luck.
Passed my IAM test about 2 years ago. Spent a while at the local club being assessed by qualified observers. My test lasted about an hour and a half and covered every aspect of driving. The tester was the top police driving instructor/tester for my area. Be prepared to give a running commentary during your test. IAM also uses the Police Roadcraft Manual and is also a charity. Once you've passed find an insurance company that recognises your qualification and ask for a quote. You'll be surprised at how good a quote you get. Big tip is observation. Good luck.