Hello Raseri, welcome to the forum.

The most recognised qualification is the level 6 diploma,(the route to which is through the certificate first) and there are a few ways to achieve this. If your employer is paying you may consider attending classes for the Nebosh Dip. which is expensive (check the net for costs) but you get face to face with a local (ish) tutor and then sit exams. A similar popular route now is the distance learning with NCRQ (again cert then dip) which you do at home and pass three modules to get the diploma. This is a cheaper option as you are not paying for face to face time, (around £1500) exams are replaced with a series of assignments (about 14) and you must pass them all to get the qualifications. Both take around 500 hrs of study time. Each route has it's fans and detractors but both are equal in value. You may stop at the certificate, many do, and go on to have very successful careers in H&S, having a diploma isn't always necessary. 

If you are already working in H&S you could take the NVQ route which assesses the work you do against certain criteria. Not sure how this is paid for or what it costs, I've never done one.

Importantly, you should brush up your research skills, this question has been asked many times on the forum, and a quick search would have told you all you need to know. Good luck.