First, a declaration of my position: I'm a Labour Party member (even if an inactive one these days) and have always voted Labour. I will certainly be voting Labour in the European Parliament election next Thursday, and almost certainly for all three of my local Labour council candidates. So read my remarks bearing this in mind.
On the European Parliament election: nationally, almost all recent opinion polls have shown Labour and UKIP as the two parties likely to get the most votes and MEPs, and I can't see anything good about UKIP "winning" the election. So voting Labour makes a bit of sense nationally, just to try to help get the Labour vote total above UKIP - unless there are good countervailing reasons in my local Euro-constituency, London.
In terms of available information, London is better off than the rest of England, as there have been three London-only opinion polls during the campaign - whose figures, however, are close enough together to give a fairly probable but far from certain result in terms of seats (Labour 3, Tory 2, UKIP 2, LibDem 1) but conflicting advice on the possibilites of tactical voting. Unlike the country as a whole, Labour has consistently been polling first in London and the Tories second just ahead of UKIP, with the LibDems in fourth place and the Greens, in fifth, some way short of re-electing Jean Lambert - though their vote has improved enough over the campaign to now make this a possibility. None of the other parties are in with a chance (and in most but not all cases, thank goodness).
In terms of movements that would change the seat distribution, UKIP will not gain its second London seat if Labour gets over twice the UKIP vote and either the LibDems or the Greens get over half the UKIP vote (Labour gets the seat from UKIP with the LibDems either keeping their seat or losing it to the Greens), or (rather less probably) if the LibDems and Greens each get over half the UKIP vote (the Greens keep their current seat). And Labour gains a fourth seat if it gets either over twice the UKIP vote (as was almost but not quite projected by the Survation poll last month) or over four times the higher of the LibDem and Green votes (as was probably just about projected in this month's YouGov poll).
I think the fourth Labour seat unlikely, though not impossible - it depends on Labour doing well at getting its vote out, which tends not to happen in European or local elections. But the possibility is still great enough to convince me to stay with Labour and not try voting tactically - particularly as, if Labour's turnout falls spectacularly, there is a remote chance that either the Tories or UKIP get a third seat instead of Labour. However, if you are more inclined to vote LibDem or Green, then it definitely improves the chances of whichever of the two you vote for keeping their current seat (though probably at the expense of the other unless they both get votes at least a bit higher than 10%).
Outside London - Scotland and Wales have their own opinion polls which can be used in the same way. In Scotland, the SNP almost certainly gets three seats to Labour's two, except in the apparently unlikely event that Labour overtakes the SNP, and the Tories get one, unless they are overtaken by either UKIP (more than a little unlikely) or the LibDems (even more so). In Wales, Labour and UKIP each look almost certain of a seat, with the other two seats probably being closely contested between a second Labour seat, the Tories and Plaid Cymru. In the rest of England, the best information available looks like nwhyte's analysis of the 2009 results and the swings needed to win or lose seats this time and the Guardian's attempt on Monday to project regional results from recent national polls, assuming uniform swings between regions. The uniform swings assumption is almost certainly flawed, but in practice the errors don't look like being above three or four per cent.
On the local elections - the "almost certainly" above is because voting Labour in my current ward is possibly letting sentiment override tactics - it is one of only three wards in Barnet which, on recent evidence, is a Tory-LibDem contest rather than a Tory-Labour one, and the LibDems (including one of their current candidates here) supported a Labour minority administration between 1994 and 2002. However, the LibDems lost their one seat in this ward four years ago, and two of the Labour candidates here are long-standing personal friends.
The borough of Barnet as a whole is an official Labour target this time - however, while I will be out campaigning all day in one of the Labour target wards (where I used to live) and I expect to see several more Labour councillors when the results come through on Thursday night, a majority would require Labour to take all the seats in a couple of wards where the best that was achieved in the late 1990s were split results. I'd love to see it, but I'm expecting the Tories to hang on with a very small majority.
So, my final plea - if you haven't voted by post already, get out and vote in the European elections today, and the local ones if you have them this year. And, in the Euros, please vote for a party that seems to have at least a plausible chance of winning at least one seat in your area - though preferably not for UKIP. For the locals, I probably don't know your area well enough to advise.