GDP data is expressed in Geary-Khamis dollars, see also UN Statistics Division definition. Obviously historical GDP data is fraught with uncertainty both as a result of data availability and methodological issues (I'm not really a fan of PPP adjustments). Also some of the countries didn't exist or territories changed during the 1820 - 2008 period. Therefore, the data is based on today's frontiers.
The following graphs shows the average GDP per capita for Western Europe including the following 30 countries: Austria, Belgium, Denmark, Finland, France, Germany, Italy, Netherlands, Norway, Sweden, Switzerland, United Kingdom (including Ireland until 1920), Ireland, Greece, Spain, Portugal, Other (Andorra, Cyprus, Faeroe Islands, Gibraltar, Greenland, Guernsey, Iceland, Isle of Man, Jersey, Liechtenstein, Luxembourg, Malta, Monaco, San Marino).
The income per capita relative to the Western Europe average for the 16 medium and large countries:
And the richest and poorest country for any given year.
Calculation details can be found here.