The snag is that Britain isn't seriously dangerous snake country - we have four species (or rather three species with one additional subspecies), plus one snake-like legless lizard, and only the adder, Vipera berus, is venomous. And it's not exactly the most dangerous species on record:
'Children, the infirm and the elderly are most at risk.'
Fatalities from adder bites are exceptionally rare. The last in Britain occurred in 1975, when a five-year-old was bitten on the ankle in Scotland. Between 1950 and 1972 there was only one death caused by an adder bite across England and Wales.
(Source Natural History Museum, https://www.nhm.ac.uk/discover/should-we-be-scared-of-british-snakes.html )
The reason for the low mortality rate is simple - they're small snakes, weighing less than 100g, really not very aggressive, and their venom is evolved for killing amphibians, not mammals. By comparison, insect stings (wasps, bees, etc.) kill five people a year in the UK, so you're roughly 150 times more likely to be killed by an insect sting than a snake bite.
So if Harry or any other wizard really wanted to be scary, he should learn to talk to and control wasps, not snakes!
This message bought to you by a story I read recently that had a character attacked by an adder that jumped at her face! Snakes do NOT jump several feet off the ground, but that's another story...
Later - forgot to say that about a hundred people a year are bitten by adders, usually after accidentally stepping on them. So the death rate is about 1 per 3000 people bitten.