Estimating the numbers of migrants in the UK is notoriously difficult, especially those with an irregular migration status who may not be counted in official government statistics. It is not for nothing that the task has been described as counting the uncountable.

However, information about the population of children and young people who are not British citizens can be essential in planning services, in promoting social inclusion, and in assisting with children’s rights claims – including regularising immigration status for those who are eligible for British citizenship.

Our report, which was published last week, profiles the population of children in the UK and London who are not British Citizens. We consider the relative numbers of foreign national children, those who are EEA+ nationals, and undocumented migrants. The report then compares this with the numbers of children and young people who have taken up regularisation pathways.

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There are several difficulties in profiling the size of the migrant population in London. Unlike some other European countries, the UK does not have national registration of residents, and so there is no official record of the numbers of migrants in the UK. Second, migrants tend to be underrepresented in census data. Although the census includes data for country of birth, this does not directly correspond to immigration status, as some British citizens are not UK-born, while foreign-born migrants can become naturalised citizens. Other survey datasets, such as the International Passenger Survey, do not include current immigration status, so it is not possible to directly count the numbers of regular or irregular migrants. Finally, despite attempts by the Home Office to introduce the ‘eborders’ system, there is no systematic means of matching people arriving in the UK with people leaving the UK, so the number of migrants and their immigration status cannot be reliably ascertained from Border Force data.

However, there are two widely used alternative options for estimating numbers of undocumented migrants. The first is the use of ‘Delphi panels’ which are a structured process of eliciting a consensus from experts in a particular field. These have been widely used as a tool for understanding migrant populations at a city level across Europe, and are particularly effective in small local areas; however, they are difficult to verify externally.

The second, and most frequently used approach in the UK is the residual method which is usually based on census data, and subtracts the known number of legal immigrants from the total foreign born population to reach a residual number which represents the de facto number of undocumented migrants in a given area. The residual method is generally seen as more methodologically rigorous than other methods, but is likely to undercount those children who have a different immigration status to their parents, and cannot be used fully in a sub-national region such as London because data is not available for internal migration.

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For our report we drew on a number of datasets and sources including the 2011 UK Census, Labour Force Survey, and Annual Population Survey, along with our previous Delphi research, and a modified residual analysis – by drawing on these different sources, it was possible to triangulate data and to make more informed estimates. We estimate that the total foreign national population in the UK at the beginning of April 2017 was 6,208,000, of whom 2,106,000 lived in London. The likely number of foreign-born children who are EEA+ nationals was 809,000 across the UK, and 260,000 in London. For EEA+ national young people, we estimated a figure of 332,000 across the UK and 95,000 in London.

We found that undocumented migrants are likely to make up a smaller percentage of the foreign-born population than has been previously estimated, mostly due to the number of EEA+ nationals legally residing in the UK since EU enlargement in 2004 and 2007. We estimate a central figure of 674,000 undocumented migrants in the UK at the beginning of April 2017, a figure which rises to 809,000 if the UK born children of undocumented migrants are included – higher than previous estimates of numbers in 2001 and 2007, which suggests that the numbers of undocumented migrants are increasing, but at a slower rate than the foreign-born population as a whole. We estimate that 215,000 of the undocumented population of the UK are likely to be children. When looking at the situation in London, we estimate that there are 397,000 undocumented migrants and their UK born children, a figure which includes 107,000 children and 26,000 young people. The chart below compares our estimate to other estimates of the numbers of undocumented children in both London and the UK as a whole.

Figure 1. Estimates of numbers the number of undocumented children in the UK and London

There are several regularisation pathways which are open to undocumented migrant children, such as applying for limited leave based on their family or private life; or registering/naturalising as a British citizen. However, not every child is eligible, and the costs of application can be prohibitive. The evidence from our report also suggests that there is a substantial gap between the total numbers of non-British children and young people and those who have actually applied. For instance, according to data from Freedom of Information requests to the Home Office, in comparison with our estimate of 215,000 undocumented children in the UK, in 2017 there were only 1,220 applications for regularisation on family or private life grounds for children, and 660 grants of regularisation under these routes. Figure 2 below shows the numbers of regularisations via the various routes.

Figure 2. Grants of regularisation of immigration status to the Home Office

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Estimating the numbers of children and young people who are not British citizens in London is fraught with both practical and methodological difficulties, and due to the nature of ‘counting the uncountable’, estimates can only ever be provisional and subject to revision. However, by using a range of methods and triangulating data, valuable insights for policy and practice can be discovered.