Nobody would deny that many (or most) of us are descended from immigrants (Danes, Romans, Angles, Saxons, Jews, Huguenots etc).
However in the past it has been measured in the hundreds or low thousands a year.
But in the last YEAR net migration in to the UK (up to March 2015) was 330,000 (net migration is the number that came in minus the number that went out).
So that is 330,000 EXTRA people in one year, a third the population of Birmingham.
If that continued for 3 years you would have 1 MILLION extra people, about the population of Birmingham.
And those 1 million need to be housed (driving up house prices), treated for illness (causing longer NHS queues), some of them educated (filling up schools), and looked after (putting up the benefits bill).
Most of those coming here have not put IN to the system (financially), but will begin to take OUT of the system as soon as they are here.
As a country we cant take that level of immigration, it puts too much strain on the whole system and infrastructure.
THAT is why people are so against immigration at the moment.
It is the NUMBER of immigrants that is the problem and it seems to be getting worse.