Besides labor is not as left-wing as PSOE is on social,police,judicial, labour and foreign affairs issues.
True, but what brought it to mind is that it is largely those left-wing voters in the UK who are sitting and not voting for the Labour party.
(A quick explanation in case anyone can't decipher the code, the issue in the UK is that under the First Past the Post system, the spread of the disenfranchised is mostly across "safe Labour seats" and so Tony Blair (for example) loses little by ignoring these people.)
The Scottish element that was feeling that way has sort of showed up in part in relation to the Scottish Nationalists. Although there are a couple of left-wing independents in the Assembly there.
I have no idea about Wales to be honest.
NI is dominated by sectarian issues.
Turnout for the EP elections is so low in general it's hard to separate out effects. 39% in 2004, but 28% in 1999. One could argue about the Green Party or the fact that the Tories + UKIP did so well...
London - I don't know the constituency breakdown well enough, but it looks to me like the effect is there somewhat.