Regarding setting up your own practice, it can be quite difficult to break into the circle. Depending on what type of work you are looking for, you will probably always get the tax return type work, but the meaty and juicy corporate project work can be difficult to get, as it will go to the reknowned names.
As an aside, the indemnity insurance can be expensive.
Yes, a lot of Solicitors, Accountants are going into tax. A number of the larger tax practices have let people go, so there have been cut backs.
Like every other walk of life, there are mediocre tax consultants but unless you have a real flair for it, I mean actually designing schemes instead of processing numbers, you would find it difficult to sustain your own practice.