I'm with Zoe, I thought at least half of us were self employed sole traders or directors of our own limited companies!
To be able to set up as a freelance bookkeeper you will need as a minimum lv2 manual and computerised accounts. Almost any good basic course will get you through lv1 but depending on your motivation and learning style you might want to take a distance learning course thereafter to be sure you have covered everything that will come up in the exams. Also, some courses come with the software needed to take the exams so there are fringe benefits to using one of the listed providers.
As to how one finds clients having got the exams and earnt ones practice licence - that is the question we all face!
I find networking fun, low cost (pick your network group carefully) and very effective. The adverts available via the practice licence page (you won't be able to see it yet) are a good source, I have a contract from there which I enjoy and the role may well grow as I start working for other businesses owened by the director.
Being self employed is not for everyone - it requires a good self starter and dedication - working from home even more so.
Good luck in whatever you decide to do.