"It's 745,000 people.
It's nearly the equivalent of a city the size of Leeds being added to the UK population each and every single year FFS."
No it isn't, because the population growth of the UK is not only impacted by the rates of immigration and emigration. Birth rates and death rates are also part of the calculation.
Over the last 5 years, the population of the UK has barely increased. Annual net growth rates are about 250,000 a year. Without the net immigration, the size of the population of the UK would be falling, with serious consequences for the economy and welfare of the country. An annual growth rate of just 0.3% represents an almost stagnant population.
Immigration is needed as the UK has an aging population. The immediate post-war cohort born around 1945 is reaching 80 years old and has not worked for the last 15 years or more. The second wave of post-war baby boomers, born around when rationing finally ended in 1954, is now approaching 70 and likewise no longer working. Immigration has been needed to fill the vacancies left when these people stopped working, and because the UK birth rate has not provided sufficient people for the job market. As these cohorts grow older somebody has to care for these people, or has to be working in order to pay the taxes and national insurance contributions to pay for their care and to pay for all of the other things that taxes pay for (education, healthcare, roads and infrastructure, policing, local government and services etc.).
Immigrants contribute more to the economy than they take out because they tend to be younger and healthier than the general population. They are also a more flexible and mobile workforce, thousands coming to the UK for seasonal jobs in hospitality, food production and agriculture - jobs that British workers don't want to do (and are actually penalised for taking due to the poor design and mismanagement of the UK benefits system) or cannot easily do due to family commitments and location. Overseas students are big business - without them, many university courses would no longer be viable.
As for the current increase in immigration, this was an entirely predictable consequence of Brexit. Having told millions of young, industrious EU citizens to bugger off back where they came from, how else was the UK going to find their replacements other than by importing labour from elsewhere? The UK economy was never going to survive the sudden loss of so many workers.