You probably won't get onto a course like speech therapy without a good set of A Levels, there are many scientific aspects to it as well as just working with people. You will need to prove that you have some academic skills.
You can become an SLT assistant without A Levels however, and maybe if you did that for a few years you would be able to get onto a course. Or alternatively go back to college and do your A Levels... there are places that do fast-track A Levels for adults where you could do them in one year.