I think you will find that if you want to be employed by any bona fide organisation these days you will have to complete the diploma. This is not a cheap option as you will have to to undergo your own counselling therapy (which you will have to pay for) and will also have to complete a minimum number of hours voluntary counselling of clients, which also will be unpaid.
The 1 year certificate course is a good foundation but if you want to be taken completely seriously, and especially to set up your own practice, I think the diploma is a "must".
Be very sure that this is what you want to do. I have known two people who have done the certificate and then the diploma, and by the time they have finished, they have felt emotionally burned out to practice, or realised that having invested the money in their training, the financial rewards are insufficient to earn them a full-time living.