Just because it's from a supermarket or petrol station means nothing.
Poundland were selling some quite frankly lethal chargers a few years back.
Whilst the current cheap chargers from Tesco (Branded Tortoise) aren't mega death-bad, having been inside one, I wouldn't be plugging it into a phone or anything I cared about either.
From a personal perspective:
I'd only use chargers that have come directly from a phone manufacturer or a large electronics wholesaler (RS Components/ Farnell)
Everything else is built to a price. Some are better than others, and plenty are lethal.
-Leezer-