| Asri Properties Ltd | Real estate | 2019 |
| Assess ALL Areas Ltd | Information & communication | 2019 |
| Asset Reliability Consultants Ltd | Professional & scientific | 2016 |
| Assettrac Limited | Information & communication | 1999 |
| Assoc of Master Tradesmen Limited | Other services | 2016 |
| Associated ASH Importers Limited | Extraterritorial | 2004 |
| Associated Lettings Limited | Households | 2004 |
| Association for Written Language and Literacy CIC | Professional & scientific | 2024 |
| Astar Roofing & Property Maintenance Ltd | Construction | 2013 |
| Astar Training and Development Limited | Administrative & support | 2008 |
| Astarita Aldrich & Ward Limited | Professional & scientific | 2016 |
| Asteria Parking Ltd | Administrative & support | 2023 |
| Aston Bush Limited | Administrative & support | 2020 |
| Aston Vaughan Lettings Limited | Real estate | 2018 |
| Aston Vaughan Sales Ltd | Real estate | 2024 |
| Astons Property Services Limited | Construction | 2008 |
| Astound Cleaning Group Limited | Other services | 2012 |
| Astra-Unifex Ltd | Professional & scientific | 2022 |
| Astral Coatings Ltd | Construction | 2025 |
| Astro Project Services Ltd | Professional & scientific | 2026 |
| Astroarch Limited | Wholesale & retail | 2024 |
| Astronox Limited | Information & communication | 2025 |
| Astrotalent Limited | Administrative & support | 2023 |
| Astute Sussex Building & Refurbishment Limited | Construction | 2023 |
| Asylum Road Ltd | Professional & scientific | 2025 |
| AT Construction Limited | Construction | 2015 |
| AT Investing Ltd | Real estate | 2023 |
| AT IT GB Ltd | Information & communication | 2023 |
| AT South Ltd | Administrative & support | 2009 |
| ATD Partners International Limited | Professional & scientific | 2015 |
| Atelier Alpine Limited | Professional & scientific | 2023 |
| Atelier Stella Ceramics Limited | Manufacturing | 2018 |
| Atelier Thirty Four Ltd | Professional & scientific | 2021 |
| Ateris Aurum Ltd | Information & communication | 2025 |
| Atermit Automotive UK Ltd | Manufacturing | 2020 |
| Atet KIN Limited | Wholesale & retail | 2020 |
| ATH Ltd | Information & communication | 2009 |
| Athanova Limited | Transport & logistics | 2025 |
| Athena Bains Ltd | Hospitality | 2025 |
| Athena Build Limited | Construction | 2026 |
| Athena Midco Limited | Financial services | 2023 |
| Athena Topco Limited | Financial services | 2023 |
| Athena UK Holdco Limited | Financial services | 2023 |
| ATI Consulting Ltd | Professional & scientific | 2010 |
| Atide Ltd | Wholesale & retail | 2022 |
| Atinus Worldwide Ltd | Wholesale & retail | 2025 |
| Atlanta Consulting Limited | Information & communication | 1999 |
| Atlantech Services Ltd | Professional & scientific | 2024 |
| Atlantic Global Scholars Ltd | Education | 2025 |
| Atlas (AHL) Holdings Limited | Financial services | 2023 |
| Atlas Bridging Limited | Financial services | 2023 |
| Atlas Engineering Investments Limited | Manufacturing | 2015 |
| Atlas PMO Limited | Administrative & support | 2025 |
| Atlas Property (Europe) Ltd | Real estate | 2003 |
| Atlingworth Court 2008 Limited | Real estate | 2008 |
| Atmani Consulting Ltd | Professional & scientific | 2025 |
| ATO Business Consulting Limited | Administrative & support | 2020 |
| Atombomb Limited | Information & communication | 2004 |
| Atomic Force Productions Ltd | Arts & recreation | 2011 |
| Atorka Health Ltd | Health & social care | 2026 |
| ATP MLI Ltd | Real estate | 2026 |
| Atrio Holdings Ltd | Financial services | 2025 |
| ATS (Travel Consultants) Limited | Administrative & support | 1988 |
| Atspeed Distributors Limited | Wholesale & retail | 1989 |
| Atspeed Group Ltd | Financial services | 2016 |
| ATT Associates Limited | Extraterritorial | 2013 |
| ATT Music Limited | Information & communication | 2025 |
| Attack Media Limited | Professional & scientific | 2016 |
| Attempt Studios Ltd | Information & communication | 2022 |
| Attercop Ltd | Information & communication | 2022 |
| Atticus Film & Television Limited | Information & communication | 2015 |
| Attract Mode Co., Ltd. | Information & communication | 2026 |
| Attuned Therapies, Consultancy, Training & Research Ltd | Health & social care | 2017 |
| Audacia Films Ltd | Information & communication | 2021 |
| Auderli Limited | Information & communication | 2019 |
| Auderli UK Limited | Information & communication | 2024 |
| Audience Management Services Ltd | Professional & scientific | 2023 |
| Audioegg Limited | Information & communication | 2018 |
| Audiography Limited | Professional & scientific | 2002 |
| Audition Health Limited | Health & social care | 2020 |
| August Post Production Limited | Information & communication | 2007 |
| Auguste and Auguste Accountants LLP | | 2023 |
| Auguste and Auguste Limited | Administrative & support | 2021 |
| Augusti Ltd | Transport & logistics | 2020 |
| Augustus Adams Ltd | Wholesale & retail | 2022 |
| Auikpartnership Ltd | Real estate | 2020 |
| AUM Alchemy Ltd | Wholesale & retail | 2025 |
| AUM Health Services Ltd | Wholesale & retail | 2007 |
| Aunty Bunny's HUT Limited | Hospitality | 2015 |
| Aura Development Solutions Limited | Real estate | 2025 |
| Aura Hair Ltd | Other services | 2024 |
| Aura Heritage Limited | Construction | 2019 |
| Aura Organic Products Limited | Manufacturing | 2019 |
| Aura Personas Limited | Other services | 2026 |
| Auralume Ltd | Wholesale & retail | 2024 |
| Aurare Ltd | Wholesale & retail | 2024 |
| Aurelia Grace Ltd | Hospitality | 2026 |
| Aurelius Productions CIC | Information & communication | 2014 |
| Aureusproperties Limited | Real estate | 2022 |
| Aurier Limited | Wholesale & retail | 2016 |