The thing with instructors is the quality can vary massively. In my experience you often find the people who would make the best instructors either don't want to do it, or they try it for a bit then give it up. There are some fantastic instructors out there, some good ones, some not so good ones, and some downright awful ones.
I think your chances of getting the job rest mainly on the turnover of drivers at your depot and whether they take on trainees or only qualified drivers. If you work at a depot with a high turnover of staff and trainees sitting around waiting for DIs, you'll generally find they'll give the instructor jobs to pretty much anyone who expresses the slightest interest, whereas if you work at a place with a low turnover the company can afford to be far pickier about who they give the job to.
Personally the instructor job has never appealed to me, mainly due to the paperwork and the politics. I also recall an instructor being put through absolute hell when his trainee had a serious incident outside of the instructors control. The instructor was subsequently exonerated but I would never wish to see another colleague be put through what he was. I'm not trying to put anyone off the role, but don't be under any illusions about it either. I'm sure it can be very rewarding and satisfying too.