| Atmore Properties Limited | Construction | 1973 |
| ATN Total Construction Limited | Construction | 2026 |
| ATN Total Consultancy Limited | Professional & scientific | 2026 |
| ATN Total Group Limited | Financial services | 2026 |
| Atob Executive Chauffeurs Ltd | Transport & logistics | 2025 |
| Atobaah Co Ltd | Other services | 2019 |
| Atom Estates Limited | Real estate | 2025 |
| Atom Surveying Limited | Professional & scientific | 2015 |
| Atomess International Limited | Wholesale & retail | 2023 |
| Atomic FX Limited | Financial services | 2025 |
| Atomical Ltd | Administrative & support | 2017 |
| Atomicmgh Ltd | Transport & logistics | 2026 |
| Atomik AM Limited | Manufacturing | 2022 |
| Atoraum Ltd | Wholesale & retail | 2024 |
| Atoun RUG Cleaning Ltd | Other services | 2024 |
| Atoun UK Ltd | Wholesale & retail | 2016 |
| Atpharma Ltd | Health & social care | 2021 |
| ATR Global Solutions Ltd | Information & communication | 2024 |
| ATR Holdco Limited | Financial services | 2017 |
| ATR Security Services Ltd | Administrative & support | 2008 |
| Atrafloor Limited | Extraterritorial | 2017 |
| Atrium Flat Management Company Limited | Households | 2006 |
| Atrium Inspection Services Ltd | Construction | 2017 |
| Atrium Talent Limited | Arts & recreation | 2006 |
| Atrytone Limited | Wholesale & retail | 2024 |
| ATS and Limited | Wholesale & retail | 2024 |
| ATT Companies Ltd | Hospitality | 2025 |
| Atta and Maui Ltd | Information & communication | 2023 |
| Attach Digital Ltd | Information & communication | 2015 |
| Attainment Care Ltd | Health & social care | 2025 |
| Attermyre Limited | Hospitality | 2003 |
| Attic Bar Limited | Administrative & support | 2019 |
| Attic Business Solutions Ltd. | Administrative & support | 2012 |
| Attic Display Limited | Professional & scientific | 2015 |
| Atticus Technology Ltd | Information & communication | 2011 |
| Attis Property Ltd | Construction | 2025 |
| Attitudes to Life Records Ltd | Arts & recreation | 2019 |
| Attm Kopuram Retail Group (UK) Ltd | Wholesale & retail | 2025 |
| Attract Properties Ltd | Real estate | 2019 |
| Attunely Ltd | Information & communication | 2026 |
| Atuk Group Ltd | Financial services | 2025 |
| Atum Developments LLP | | 2008 |
| Atuun Ltd | Wholesale & retail | 2026 |
| Atwbuilders Ltd | Construction | 2025 |
| Atwell Interiors Ltd | Wholesale & retail | 2025 |
| Atwell Trading Ltd | Wholesale & retail | 2022 |
| Atwood GAS Services Ltd | Professional & scientific | 2020 |
| AU Aesthetics Ltd | Other services | 2018 |
| AU Gateway Global CIC | Administrative & support | 2025 |
| Aubrey Morgan Limited | Administrative & support | 2022 |
| Aucapo Limited | Information & communication | 2023 |
| Auckland College Holdings Ltd | Education | 2019 |
| Auckland College Ltd | Education | 2010 |
| Audax Group Ltd | Construction | 2019 |
| Audax Properties Ltd | Construction | 2024 |
| Audere BW Architecture Ltd | Professional & scientific | 2019 |
| Audi and BMW Dealer Limited | Wholesale & retail | 2025 |
| Audiogo Limited | Information & communication | 1977 |
| Audiolux Limited | Wholesale & retail | 2012 |
| Audire Care Limited | Extraterritorial | 2022 |
| Audit and Stock Control Ltd | Administrative & support | 2003 |
| Audlem Developments Ltd | Construction | 2022 |
| Audley House Ltd | Financial services | 2023 |
| Audsley Holdings Limited | Construction | 2022 |
| Auger Site Investigations Limited | Water & waste | 1995 |
| Aughton Automation Limited | Professional & scientific | 1999 |
| Aughton Estates Ltd | Real estate | 2011 |
| Aughton Lane Ltd | Professional & scientific | 2017 |
| Aughton PIG Producers Limited | Agriculture & fishing | 1991 |
| Aughton Wood Management Limited | Households | 2019 |
| Augment Corporate Services Ltd | Administrative & support | 2025 |
| Augustus Resort Owners Ltd | Real estate | 2009 |
| Auld Alliance Actuarial Ltd | Professional & scientific | 2019 |
| AUM Traders Ltd | Wholesale & retail | 2025 |
| Aura Apartments Liverpool Ltd | Real estate | 2023 |
| Aura Assisted Living Limited | Health & social care | 2025 |
| Aura Capital Trading Ltd | Financial services | 2026 |
| Aura Clarges Ltd | Real estate | 2023 |
| Aura Hair Salon Ltd | Other services | 2011 |
| Aura Infection Control Limited | Health & social care | 2013 |
| Auramarinetech Ltd | Information & communication | 2025 |
| Aurdxxa Limited | Wholesale & retail | 2026 |
| Aurea Tactus Ltd | Construction | 2025 |
| Aurello Technologies Ltd | Information & communication | 2026 |
| Aureolin Capital Limited | Real estate | 2020 |
| Aureus Solutions Ltd | Professional & scientific | 2026 |
| Aurevesta Ltd | Wholesale & retail | 2025 |
| Aurevo Activewear Ltd | Wholesale & retail | 2025 |
| Aurgenis Limited | Professional & scientific | 2026 |
| Auri Consulting Ltd | Professional & scientific | 2019 |
| Aurion Digital Ltd | Information & communication | 2020 |
| Auris Living Ltd | Hospitality | 2025 |
| Aurivana Ltd | Other services | 2025 |
| Auriviva Ltd | Real estate | 2026 |
| Aurora Creative Agency Ltd | Information & communication | 2020 |
| Aurora Developments NW Ltd | Construction | 2025 |
| Aurora Education Ltd | Education | 2025 |
| Aurora Potential Limited | Wholesale & retail | 2010 |
| Aurora Property Finance Limited | Financial services | 2009 |
| Aurora Yarn Limited | Manufacturing | 2025 |