In the blast zone
Southend's population is around 175,000. I live a few yards from the estuary on the fringes of the predicted explosion. We would certainly lose our windows to the air blast (I think most houses in town would) and would definitely be swamped by the tidal wave. There is the possibility of further smaller explosions - gas mains, boilers etc and whether there would be the wherewithal to fight these fires. Local emergency services would struggle to cope. The power would likely be out for days. Sheeness would get it even worse.
Then there the possibility of the oil refinery going up, what would happen to any container ships heading up and down the estuary channel to Tilbury Docks and what the long-term economic impact would be in the aftermath.
Nobody wants to play Jenga with the wreck but irresponsible thrill-seekers who want bragging rights do go out to it. They are gambling with more lives than just their own.