On April 23, 2016 ukpublicspending.co.uk updated UK GDP and public debt outturn and forecasts released with the HM Treasury Budget in March 2015.
Recent and forecast GDP comes from the UK Office of Budget Responsibility's Economic and fiscal outlook report, using tables 1.1 and 1.2 in a spreadsheet file labeled "Economic and fiscal outlook supplementary economy tables". UK GDP previous to 2010 comes from Measuring Worth. The data goes back to 1692.
Public sector net debt, outturn and forecast, comes from a spreadsheet called "Aggregates (£bn)" in the Public Finances Databank at the Office of Budget Responsibility.