This shows the combined voting share of Labour/Conservatives, in the general elections and European elections since 1979 (when direct European elections started). Blue is general, Red is Europeans.
There is a notable tend, in that while the Lab/Con share for European elections starts higher than generals, it ends up much lower. For European elections from 1979-1994, large single-member constituencies were used, making the system even less proportional than the system used for general elections.
There's a consistent downward trend for the European elections, but it starts seriously dipping below the general election figures in 1999, the first time PR is introduced. The next time round in the Europeans the combined vote share of Labour and Conservative drops below 50%, and falls even further in 2009. Doesn't it look like voter behaviour is aware of the electoral system and changes accordingly (if with a little lag)? Yet the media appears to treat these vote shares as comparable.