I am seriously considering doing a phd as I enjoy research. But what will be the main advantage of doing a 3 yr phd over simply going for a research assistants' job for more money?

If you want to progress in academia then you need to have a PhD. Working as a research assistant will not help you to progress in the same way. The best option is to get a research assistantship with a supervisor who will also let you register for a part time PhD. That way you get the money and the training (usually 'part time' PhD's of this nature take the same time as an ordinary PhD, but as you are in employment you can't be registered as a full time student).

Remain a student for three more years (think, no council tax, no income tax, student discount everywhere!!). Oh, and if you enjoy research, it is worthwhile, a PhD is what you make of it, they are tough, frustrating and in some cases boring for a while. But they are also very rewarding as it is YOUR research