A person from country C can immigrate into the UK by immigrating into one of 27 different other countries, and then using free movement to get into the UK. This makes border control harder.
The UK manages to both have influence over EU immigration policies AND also have exemption to it for non-EU immigration, AND can apply its own limitations even for when immigrants get citizenship for a limited period. Yet many brexiters spout on about how the EU just dictates everything to us and it's all out of our control. Absolute rubbish.