The Home Office publishes data on people in detention in the ‘Immigration Statistics Quarterly Release’. https://www.gov.uk/government/collections/immigration-statistics-quarterly-release
Data on the number of people detained under immigration powers, broken down by nationality, age, and length of detention are published in table Det_D02 of the immigration detention detailed datasets. https://www.gov.uk/government/statistical-data-sets/returns-and-detention-datasets#immigration-detention
Information on how to use the dataset can be found in the ‘Notes’ page of the workbook. The latest data relates to the year ending March 2020. Additionally, the Home Office publishes a high-level overview of the data in the 'summary tables'. The ‘contents’ sheet contains an overview of all available data on detention.
A statistical report Statistics relating to Covid-19 and the immigration system, May 2020, https://www.gov.uk/government/statistics/statistics-relating-to-covid-19-and-the-immigration-system-may-2020released on 28 May 2020 provides further high-level information relating to detention and Covid-19 up to the end of April.
Figures covering the second quarter of 2020 will be released on 27 August 2020. Information on future Home Office statistical release dates can be found in the ‘Research and statistics calendar’. https://www.gov.uk/search/research-and-statistics?content_store_document_type=upcoming_statistics&organisations%5B%5D=home-office&order=release-date-oldest