Hi,
I am doing the RHS general cert. I started at the local college in September. It is a great course, so far anyhow, and it would certainly appear that if you want to do gardening professionally then it is a recognised qualification which is highly regarded. Not sure if it would alone lead you to a career -you would probably need to embark on the advanced course but if you do the general and then start at the bottom and get experience somewhere such as an RHS garden, maybe this would lead you further into a career? I hope this is of some help. ;D