It really isn't complicated, you just phone up and ask for it. You don't have to re-claim it every year. Joining as an S&T trainee, I claimed the allowance for having to provide my own workwear (this was in the days before company-supplied HiVi and safety boots), and was still getting it nearly 20 years later until I left BR.
It is worth checking your tax code each year, particular at significant life events, such as marriage, change of job, retirement, etc. If they get it wrong, it is usually because either you or your employer has not given them significant information. For example, when I was TUPE'd from one job to another, my new employer told them about the transfer, but my old employer didn't, leading HMRC to think I had two jobs and double the income!