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Billy Webb's Amazing Story? Can’t find very much information about this show online but the Wikipedia page says “The villain is played by a different actor each week“ (so presumably a shapeshifter, though it doesn’t quite match your description)
and this synopsis [1]
of the last episode says the boy is setting a trap
Billy Webb's Amazing Stories sounds like a great shout. It sounds like it shares many aspects, and there's even an ice cream man listed in the credits of the finale. I don't recall it being told after the event to a reporter though and it doesn't *feel* like the right show (though my memory of a show 27 years after the event isn't going to be perfect). I'm trying to find the final episode as that would clinch it either way
Comment on this video reckons there was a similiar programme around the same time which involves a magic ring. I'm sure I've seen another comment somewhere about a magic ring in my searches. I can't say either way if there was a magic ring in the one I'm thinking of. Also can't find the final episode of BWAS.
https://www.youtube.com/watch?v=xlPaw-475w0
Sorry, I was a bit rude in my latest reply. I know how it feels to want to find out something for so many years. I have been searching for a book about a girl and a talking tree (or trees) for so many years and was never able to find it. My folks had it, but it was before we moved, so I do not know if we still have it, hope we do, I would love to finally solve it. I appreciate you took my advice about searching on Reedit as well, they are helpful there too. I hope you find this. Good luck mate.
Edited: obviously, it is not Terminator 2 - Judgement Day, I just thought it might make you find your answer quicker. As I suggested, perhaps something similar. I am sure you will find this, just keep on searching.
Edited by thestormtrooper

I'm struggling to remember a UK children's afterschool programme, probably CITV though possibly CBBC which I think would have aired very early 90s.
There's a bad guy who can shapeshift/change his appearance to look like anyone else. I think this really meant that he appears on-screen as the same man, but none of the other characters can tell it's the same one except a boy who isn't believed when he points this out to others. I'm pretty sure he appears as an ice-cream van man at least once, maybe this is his default persona, but appears in different clothes to appear as different men. I think once, the boy runs upto someone such as a policeman/park attendant to tell him and then realises that the man he's telling is actually the bad guy.
I can't recall what the bad guy's scheme is, but I remember the ending where the boy and man are in a park. The kid is trying to lure him onto a (turned off) water fountain as for some reason, water kills/dissolves him. Realising the plan, the man confidently jumps onto the fountain saying it's turned off (probably for the winter) so he'll be ok. The fountain then turns on and the man is vanquished.
It's stumped me for ages. Can anyone help?