I don't think the UK should be accepting immigrants right now because our essential services are already badly overstretched.
Hospitals - serious bed shortage, long waits for operations. I've spent a lot of time in hospitals lately and the over stretching of resources/staff is terrifying.
Schools - oversubscribed, kids stuffed in every little nook of the existing ones. Teachers have too many kids to teach.
Housing - in my city alone, 13,000 families are waiting to be housed. 
Police - insufficient resources to investigate most crimes (eg burglary), unless very serious.
Roads - frequently gridlocked.
Public transport - frequently ram jam full.
I just don't get it
. We cannot manage with our current population size and density. How can we manage with more?