Some biomass numbers

Miscanthus harvesting

Energy crops and wood chips/pellets can be burnt for heat or to generate electricity. Miscanthus yields about 15 tonnes per hectare and 4,200 kWh/tonne gross or 1,680 kWh/tonne net (at 40% efficiency). This is 25,000 kWh per hectare per year. According to David MacKay's Hot Air, each person in the UK uses approx 18 kWh (of electricity) per day or 6,570 kWh per year. Therefore each hectare of Miscanthus provides enough electricity for about 4 people.

The area of the UK is approx 24M hectares so we would need about 58% of the UK planted in Miscanthus to provide current electricity use.

Update: primary energy consumption per person in the UK is about 125 kWh/day (MacKay again). This includes all fossil fuels + wind + hydro. But not electricity generated from fossil fuels? NB: this would need 2 hectares of Miscanthus per person or 500% of UK land area!

Update 2:
Google's data browser has the World Bank showing UK electricity consumption at 6,122 kWh per person per year, or 16.8 kWh per day.