This has always been metric. The unit of power, the watt, is named after British scientist James Watt, and all electricity pricing is in pence per kilowatt hour.
This means that for every appliance that uses 1 kilowatt of power for a period of 1 hour, that price in pence is what it costs to run that appliance, e.g. if the cost is 10 pence per kWh, and the appliance uses 1 kW and is used for 1 hour, then the cost is 10 pence. If it uses 2 kW, then the price doubles to 20 pence in 1 hour. If it uses 100 watts, which is one tenth of 1 kilowatt, then it uses 1 penny per hour (such as a light bulb).
Or if you have 5 light bulbs at 100 watts each, running for only 3 hours each day, for a whole year, then the cost per year would be: 10 pence × 5 bulbs × 0.1 kW × 3 hours × 365 days = £54.75.
Other metric units of electricity include volts, amps, ohms and farads.
Gas bills are also now produced with costs in pence per kWh, which makes it very easy now to compare gas costs with electricity costs.