Internet says: "A standard cord of well-seasoned hardwood (stack of wood 4'X 4'X 8' or 128 cubic feet) contains the heat equivalent of about 20 million BTU's. By way of comparison this is more or less equivalent to the heat value in 145 gallons (3.38 barrels) of #2 fuel oil or 215 gallons of LP gas (5.119 barrels)."
As a rule of thumb, you can sustainably harvest one half to one full cord (128 cubic feet or 4'x4'x8′) of firewood per acre per year.
1 short ton (2,000 pounds) of coal (consumed by the electric power sector) = 18,820,000 Btu. Therefore, an acre of forest is like a tonne of coal a year.
Great Britain used 75 to 275 million tonnes of coal per year 1860-1980 (More precise figures in a spreadsheet here tell the same story: https://www.gov.uk/government/statistical-data-sets/historical-coal-data-coal-production-availability-and-consumption)
Great Britain is 60 million acres, so even at 100% forestation it wouldn't equal coal, but could contribute.
delendi sunt*