I am answering your question from my own experiance, once you complete your level 3 you can apply for your practice licence.
Once you complete the level 4 self assessment, ICB add that to the services you can offer and you can offer self assesment services to clients.
I have completed the self assessment and i found it difficult, it is a lot to take in and takes a while to get through it.
I am currently working through the level 4 finacial statements for small/micro companies as this is best completed before the corporation tax module.
All the best for completing your level 3 and getting your practice licence.