Recent immigration is the result of of the UK's decision to allow nationals from so-called A8 countries - the east European and Baltic states that joined the EU in May 2004 - access to the UK labour market.
This recruiting of foreign workers, is mainly to fill gaps in sectors and jobs in which people are unavailable or unwilling to work and which cannot be transferred abroad, such as agriculture, catering, cleaning and some building work. In Britain the issue of work permits to employers, enabling them to recruit workers from abroad for specific jobs, has nearly doubled since 1998, and other legal routes to enter Britain to work have been opened.
Immigrants make large contributions both to economic growth and to public finances, since they are mostly young, fit and educated at others’ expense. Most, if they are legally permitted to and sometimes if they are not, are willing to work for long hours and in poor conditions in jobs which do not require their qualifications.
There is one possible economic rationale for immigration controls, which is that their existence makes immigrant workers precarious, and therefore more exploitable.
You, by backing immigration controls, unwittingly support this increased exploitation on workers.
For more on this see:
http://www.noii.org.uk/