You cannot become a dog trainer just by going on a course & then start training other people’s dogs or other people to train their dogs(although some people seem to think so)
You really do need to have owned a dog & trained it.
If you live in the midlands there are lots of dog training cubs who train their members to become dog trainers, but if you are looking to become a professinal dog trainer you have as others have written in this thread to gets lots of experience of dogs)
M friend who has been a professional dog trainer for years(she trains other peoples dogs & rescue dogs etc)says the dog training is easy, it’s training people to train their own dogs that is hard. You need to be able teach people if this is what you want