A 57% charge on a Model Y Long Range would be 188 miles of range (US EPA estimate). UK petrol prices are £1.54/litre or $7.03/US gallon. Assuming a 30-MPG car, 188 miles would be $44. They paid $39.59 for that distance of electricity. Hybrids and plug-in hybrids will do a lot better than 30-MPG so the Superchargers aren't cheap compared to other available options. The average UK petrol vehicle gets over 35-MPG (6.7l/100km) so that would come in at $37.76 for the 188 mile range and cheaper than the Supercharger for an average petrol car, though it can be slightly hard to compare since different countries have slightly different fuel economy evaluations.
It is higher than California by a decent amount and home energy rates are high too. They're a lot cheaper than petrol, but at 22.8p ($0.277) per kWh, it's nearly double what I'm paying on the east coast, but it looks like California rates might be comparable.
It's not outrageous, but it is definitely higher than California and can mean that the Superchargers are more expensive than petrol stations.
Sources: https://electrek.co/2022/09/28/tesla-hikes-supercharger-pric..., https://www.carmagazine.co.uk/electric/tesla-supercharger/, https://www.iea.org/articles/fuel-economy-in-the-united-king..., https://powercompare.co.uk/electricity-prices/