Sometimes, you've got to laugh.
Of course, all the ads I've seen have been in English-speaking countries, so there is a difference. Not that Brits can't be xenophobic towards Americans or Australians, of course, it's just less common. The other fun thing is that last summer we had the Olympics, which always serves as an advert for the hosts, and probably one with greater penetration than any conventional campaign. And they say government is disorganised.
More seriously, this seems to be part of a general trend recently where Cameron is being increasingly anti-Europe. The pledge for a referendum over EU membership is one obvious example. It seems to me that there are two things causing this. First up is the unhappiness within the Tories over the current government. While he has denied it, Adam Afriyie MP was rumoured to be plotting to oust Cameron, not a healthy sign.
However, a bigger (and somewhat related) fact is probably found by looking at the polls. UK Polling Report puts Labour up by 9 in an average of polls, despite Ed Miliband not being especially popular. And the sad truth is that a quick bit of foreigner-bashing is likely to be popular, especially with swing voters. Which is sad, and ironically something that supports the negative ads.
One of the interesting things about the EU in general is the freedom of workers to move around. Generally free trade agreements remove restrictions on the flow of capital, while being much stricter about the flow of labour. This allows those of us in rich countries to enjoy the benefits of things being produced at low cost (low wages) in a largely unequal manner. Of course, even in Europe we see that some countries (Germany) have benefited more that others (Greece); even when you remove legal restrictions, it is harder for labour to cross borders since it involves a fairly major upheaval.
So Britain does have a self-interest in restricting European immigration. (It's just a pity it so often manifests itself in borderline or outright bigotry.) The higher standards of living in the UK and the stronger (if weakening) social safety net are obviously attractive. Higher immigration means both lower wages and higher social expenditures, whereas the gains we make from European Free Trade are more intangible. So in that sense, a self-deprecating ad campaign could easily be worthwhile. But it still amuses me.