There are a mine field of courses on-line. I am sifting through them. It would be could to get feedback or hear from others who are experienced or looking for courses.
A great way of finding out about courses and training is to volunteer or become an intern at a public garden. Kew, the RHS and the National Trust all offer opportunities. To apply for an internship you usually need a little horticultural experience, so it's often a good idea to volunteer first. Higher education colleges such as Bicton often have open days where you can put your questions directly to the staff. They'll be quick to work out the level of course that's right for you.