It appears that wind stations south of Scotland produce their name plate capacity for about 25% of the time. Some do worse, but that seems like a reasonable figure. (In Scotland they go over 30%.)
http://www.democraticunderground.com/discuss/duboard.php?az=view_all&address=115x75583http://www.telegraph.co.uk/news/main.jhtml?xml=/news/2006/12/09/nwind09.xmlIt may be that there are few times that the output reaches the rated capacity, but these are balanced by time when no power at all is available.
If we assume that these plants will operate at the same 25% capacity the plants in Cumbria operate, even if they are on the other side of the island, they will really produce energy equivalent to a 0.32 gigawatt or 320 MW gas plant. The actual energy benefit may be smaller depending on British policies for the maintainence of "spinning reserve."