Here are the anti-immigrant arguments that have been made so far on this thread:
We'll be 'swamped by the Third World' - effortlessly dealt with by FAQ, who pointed out that most immigrants are not from developing countries. (I would like someone to explain to me why anyone should worry more about immigrants from developing countries than immigrants from, say, Eastern Europe.)
The UK is a 'small country' and we 'don't have enough room': batted away by explaining that in terms of population density, we have some way to go before we're leading the world.
'We can't have people coming here to live off benefits' - as Expat has just ably and amply explained, they don't.
'It's a strain on social services provision' - it's not. Economic migrants bring in far more in tax revenues than they take out in social services. Where local services are stretched too far, it's the fault of the government.
'The UK is seen as a soft touch' - not any more it's not. We've recently instituted some of the nastiest anti-asylum seeker standards in Europe - in fact we've been censured for it by the UNHCR, IIRC. (See FAQ's point that Zimbabwe is deemed 'safe'.) Our standards for economic migrants are no softer than any other European country. Numbers of immigrants and asylum seeker applications are falling.
Furthermore, if anti-migration posters want a reasoned debate, they have to start distinguising between economic migrants and asylum seekers, because they are two very different things, and the arguments are correspondingly different.
I'm all for listening to people's arguments on this topic. Doesn't stop me knowing that most of the anti-immigrant arguments are desperately ill-informed. Democracy is not best served by listening to ignorant opinions and promising to take them seriously.