Yes, that's £0.36 per day. Incidentally, you brits sure pay a lot for electricity. Here in the states, it's less than half that.

250 watts = 0.25 kilowatts, NOT 0.25 kilowatt hours. big difference. kilowatts are a unit of power, like watts. kilowatt hours are a unit of energy. power is the rate at which energy is used.

Unless you have a really crappy ballast, it probably generates about 50 watts of heat, making your total lamp usage more like 300 watts. This makes the cost more like £0.43 per day.