It's more of a "get your shit charged up until you have time to buy an adapter" kind of tip, but it actually is pretty safe. Only problem is that UK sockets are designed to accept square shaped plugs and won't make a perfect contact with round shaped plugs, if you're using a high powered appliance like a kettle or a heater that could cause the bad contact to arc or get overheated and start a fire but for low powered chargers etc. it's pretty safe. Electrocution risk next to none as long as it's done with a non conductive tool. That contraption right there powered my laptop for almost a year.
“Our great danger is not that we aim too high and fail, but that we aim too low and succeed.” ― Rollo Tomassi