This information is available as a link from the Electricity Data Summary Page (https://www.bmreports.com/bmrs/?q=eds/main) on the bmreports.com BMRA web site, along with indicative generation data broken down by fuel type for the previous 3 months. The link to the spreadsheet can be found below the graph headed "Generation By Fuel Type"
Data older than this is not retained by the BSC Central Services provider.