In 2019, the population of the whole of the UK was estimated to be 66,796,807. This is an estimate, because unlike some other countries, the UK does not have a population registry.
In countries with a population registry, the government collects data on all its residents. Residents are usually obliged to notify the government of any changes in their status such as changes of address, residency, or marital status. These countries usually also have a compulsory ID card for each resident, to keep the register up to date. Contrastingly, the UK government has no such database or register. General public opinion is against the introduction of such a system, as it is considered too intrusive and infringing on the right to liberty. Instead, the Office of National Statistics in the UK assumes a number for the current population, based on what is known about births, deaths and population ageing combined with expectations based on historical data about levels of immigration.
As a consequence, it is quite hard to estimate how many migrants come and go. This is the case both for regular migrants, especially those from the EU or other visa-free countries, but even more so for undocumented migrants.
Overall, the ONS reports that international migration in the year to mid-2019 was 16% lower than in the previous year. This was a result of 17,000 fewer immigrants (a 3% decrease) arriving than in 2018, and 28,000 (or 8%) more people emigrating. This downward change is estimated to have continued throughout 2020 as a result of the COVID-19 pandemic and the finalisation of Brexit. Some have said that the compound effect of COVID and Brexit have led to an “exodus” of foreign-born people from the UK, with some estimates reporting that 1.3 million people have left the UK since the end of 2019 (https://www.escoe.ac.uk/estimating-the-uk-population-during-the-pandemic/). But the numbers are not known, and impossible to confirm.
One illustration of how imprecise the numbers are exactly is the estimated amount of irregular or undocumented migrants in the UK. Preliminarily, it is important to know that until 30 June 2021, these statistics comprise only non-EU citizens, as immigration rules will not be enforced against EU citizens until after that date. Focusing on non-EU migrants in the UK, then, there are no accurate data on the number of lawfully resident non-EU citizens. The Home Office is required to report yearly estimates of the number of non-EU citizens holding a valid residence permit. They do this based on the number of temporary residence permits issued that year that remain valid. It includes for example, visas for entry and extensions where these have been granted. However, the estimate does not include migrants who have been granted permanent residence in the UK (i.e. indefinite leave to remain, or settled status). To make things worse, the government also has no way of knowing if those who were previously granted indefinite leave to remain still live in the UK now, or whether they are even alive. Any estimate of the migrant population, therefore, requires numerous assumptions to be made based on average yearly ILR grants, birth rates, death rates, etc. It is likely to have a significant margin of error for migrants with lawful residence, let alone for those without it.
Another recent example of how difficult it is to estimate population numbers is the Home Office failure to estimate the number of EU citizens living in the UK. Since 1 January 2021, free movement for EU citizens has ended, meaning they can no longer move to work and live in the UK as they please. EU citizens who lived in the UK before that date can apply to retain their residency rights under the EU Settlement Scheme. When the Home Office launched the EU Settlement Scheme in 2018, they estimated that three million EU citizens were living in the UK, and that this was about the number of applications they would receive. With over five months left to apply, over four million people have applied to the Scheme. In the five months to come before the Scheme closes on 30 June 2021, thousands if not a million more are expected.
In short, the government does not know exactly how many migrants there are in the UK, and has no real way of finding out unless it introduces a formal registration system like in the Netherlands or Belgium.
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