I've blogged before on tactics that opponents of immigration use to portray themselves as motivated by factors other than racism, including faux concern for the working man (but only the British-born working man) and lies about pressure on public services.

Another popular one is "we'd love to take more people, but we're full".

An obvious retort to the definitely-not-racist [*] person raising this point is "err, the UK has the 51st highest population density in the world at 255 people/square km, behind Belgium, India, the Netherlands and the Philippines, and barely any higher than Germany or Italy". This can be suffixed with ", you dolt" if required.

This is unlikely to convince most not-racist types.