I think there is also a "public interest" issue as well.
If the suspect is unknown then the media will not be interested and no publicity follows.
Targeting those in the media is possibly done even if there is a risk of failure as the media attention gives the HMRC free publicity.
It is a very complex area: my accountant is paid to make sure that we pay all the tax we should as i never, ever want HMRC on my case.
Mind you they don't help themselves: A couple of years ago I had three bills stating I owed "No pounds and no pence" and a final demand threatening prosecution if I didn't pay. I wanted to send a cheque for the sum, but my accountant said no, ignore it. It seems that it was a computer issue and only when a person reviews the case is it then cancelled.
