Dylan is now 26 months - today!! - and I have opted for giving him follow on milk. I just thought that the follow on milk has extras that are good for him, in that way, better in my opinion than whole milk. I have not done so by anyones advice (HV, GP or anyone else), I just thought that I found a milk that he seems to enjoy and saw no point in changing it. Follow on milks say they suitable until 36 months, and I much prefer them.
Also, Dylan suffered from NEC, and though completely cured from it now, I still think follow on formula milk is better for him than formula.
Hope this helps