system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: What is UKIP and why are they controversial?
gpt
A political party in the UK - The United Kingdom Independence Party.

Their policies are fairly protectionist - they want security for British people and workers. As a result of this, they want immigration to be vastly reduced and more controlled; for instance, only skilled workers in industries where the UK lacks the necessary workforce.

This is (allegedly) to protect natural British citizens. 

This is also (one of) the reasons they are controversial. How can they define 'natural British citizen'? Is a child born in Britain to immigrant parents a natural British citizen? 

Also, their policies are ripe for more unsavoury members of society to get behind them and become active. Lost your job to a Pole? "No, you're not overvaluing your skillset - it's the Pole's fault. None of this could happen if we were out of the EU - join UKIP".

TL;DR, they are controversial for many reasons - not least because they attract racist voters (and probably have a significant number of racist members).