Immigration is when people from other countries come to the UK.
historically there have been waves of immigration into the U.K. - eg in 1685 and beyond many many French Protestants left France for the UK and other Protestant countries.
https://en.m.wikipedia.org/wiki/Edict_of_Fontainebleau
more recently after the Second World War the government encouraged immigrants to move to the U.K. because we had a massive labour shortage. I grew up in Lancashire and many Pakistani people moved there in the late 40s and early 50s to work at the Cotton mills.
at the time commonwealth citizens had the right to live and work in the UK so strictly speaking they were British citizens born abroad who moved to the UK rather than immigrants. A lot also came from the carribean - the wind rush generation
What is Windrush Day? https://www.bbc.com/news/uk-43782241
in 1981 British nationality was redefined by the British Nationality Act and citizens of current and former colonies such as Jamaica, Pakistan etc were now considered mostly not British citizens.
Immigration stayed pretty steady through the 80s and started going up in the 90s.
https://www.statista.com/statistics/283287/net-migration-figures-of-the-united-kingdom-y-on-y/
the big changes came with free movement in the EU in the early 2000s when many people from Eastern Europe (Poland etc) moved to Britain.
since Brexit many of the EU citizens who were here have left and immigration has gone up massively and is now mostly from non-EU states such as India and Pakistan.
why people don’t like immigration:
it can put a massive amount of pressure on local government services in a particular area and government isn’t always good at meeting that need.
for example many schools in the fens had an influx of polish children when free movement started who spoke no English at all. There was no support from government for these children, schools just had to do their best with random 11 year olds who had no English.
Immigrants tend to cluster together where there are already people from their country (for obvious reasons). If you are English and your village switches very quickly to being a place where many people speak Romanian or similar and no English it can feel alienating
economically there is an argument that high immigration keeps wages low because there are more people around to do the jobs.
there are concerns about some of the cultures that immigrants are coming from - some asylum seekers in particular almost be definition will come from places of extreme violence and significant cultural differences (for example Afghanistan) and they may find it difficult to adjust to English cultural norms.
why people like immigration:
polish builders are amazing
immigrants mostly do jobs British people can’t or won’t eg veg picking and harvesting
the UK has always been a multicultural society and the skills and experiences immigrants have brought have enriched it - eg Notting Hill carnival, Birmingham Mela, etc.