| Atomic Projects Limited | Information & communication | 2025 |
| Atomicgtm Ltd | Professional & scientific | 2023 |
| Atomium Films Limited | Information & communication | 2007 |
| Atrebatum Limited | Professional & scientific | 2005 |
| Atreides Management UK Limited | Administrative & support | 2019 |
| ATS Digilabs Ltd | Wholesale & retail | 2024 |
| ATS Property Solutions Ltd | Real estate | 2025 |
| ATT Consulting Solutions Ltd | Information & communication | 2022 |
| Attar Aroma International Limited | Wholesale & retail | 2025 |
| Attempt Solutions Limited | Information & communication | 2023 |
| Attention Productions Limited | Extraterritorial | 2013 |
| Atterburies Limited | Real estate | 2018 |
| Atterbury Property Developments Ltd | Construction | 2019 |
| Atti (ME) Limited | Professional & scientific | 2019 |
| Atti Consultants Limited | Professional & scientific | 2002 |
| Atti International Limited | Financial services | 2004 |
| Attics International Ltd | Professional & scientific | 2025 |
| Attila&C Ltd | Administrative & support | 2019 |
| Attitude Accessories Limited | Wholesale & retail | 2004 |
| Attitude Aerobatics Limited | Arts & recreation | 2008 |
| Attitude Group Ltd | Information & communication | 2023 |
| ATTIX5 UK Limited | Information & communication | 2001 |
| Attune Finance Limited | Professional & scientific | 2016 |
| Attune UK Limited | Information & communication | 2005 |
| ATW Holdings Limited | Financial services | 2015 |
| Atyip Property Limited | Administrative & support | 2021 |
| AU Courant I T Solutions Ltd | Information & communication | 1994 |
| Aubade Limited | Professional & scientific | 2009 |
| Aubas Ltd | Real estate | 2022 |
| Aubergine Music Ltd. | Information & communication | 1997 |
| Auburn Jungle Limited | Professional & scientific | 2010 |
| Auctus Property Limited | Real estate | 2023 |
| Auctuspro UK Limited | Real estate | 2024 |
| Audiolibrix Ltd | Wholesale & retail | 2012 |
| Audiotek Ltd | Professional & scientific | 2015 |
| Augralux Consulting Ltd | Information & communication | 2026 |
| August Hugo Bratstrom EX Ltd | Wholesale & retail | 2025 |
| Augusta Property Services Limited | Real estate | 2024 |
| Augustus & Augustus UK Limited | Wholesale & retail | 2024 |
| Auldham Limited | Professional & scientific | 2014 |
| Aulnage Limited | Wholesale & retail | 2014 |
| AUP Consulting Ltd | Information & communication | 2015 |
| Aura 6 Group Ltd | Wholesale & retail | 2026 |
| Aura Software Ltd | Information & communication | 2021 |
| Aura Vocals Elite Ltd | Information & communication | 2026 |
| Aurantus Ltd | Extraterritorial | 2023 |
| Auray Key Ltd | Administrative & support | 2025 |
| Aurea Beauty Limited | Other services | 2025 |
| Aurea Estates Ltd | Real estate | 2025 |
| Aureum Digital Ltd | Information & communication | 2014 |
| Aureum Partners Limited | Wholesale & retail | 2025 |
| Aurora Asset Managers Ltd | Real estate | 2025 |
| Aurora Controls Ltd | Construction | 2023 |
| Aurora Family CIC | Education | 2020 |
| Aurora PDR Limited | Administrative & support | 2002 |
| Aurora Tattoo Emporium Ltd | Other services | 2023 |
| Aurosó Limited | Manufacturing | 2019 |
| Aurras Ltd | Manufacturing | 2020 |
| Aurum Solutions Limited | Information & communication | 2004 |
| Ausm Limited | Wholesale & retail | 2019 |
| Ausonia Services Limited | Administrative & support | 1994 |
| Auster Housing Limited | Real estate | 1995 |
| Austin Clark Recruitment Limited | Administrative & support | 2008 |
| Austin Oliver Properties Limited | Real estate | 1953 |
| Austin Worboys Construction Ltd | Construction | 2026 |
| Austin Worboys Ltd | Construction | 2021 |
| Aut-Ai Limited | Information & communication | 2022 |
| Autangel Community Interest Company | Health & social care | 2013 |
| Authentic Business Associates Ltd | Other services | 2024 |
| Authentic Consulting Ltd | Professional & scientific | 2024 |
| Authentic Creative Catering Limited | Hospitality | 2026 |
| Authentic Duplicate Ltd | Information & communication | 2026 |
| Authentic Health Care Limited | Health & social care | 2017 |
| Authentic Recognition Ltd | Administrative & support | 2025 |
| Authentic WOW Ltd | Administrative & support | 2016 |
| Authocracy Ltd | Information & communication | 2020 |
| Authorn Limited | Construction | 1977 |
| Authorn Surfacing Limited | Construction | 2008 |
| Autism Mentors Limited | Education | 2017 |
| Auto Assist Mechanical Services Ltd | Wholesale & retail | 2015 |
| Auto Commercial Limited | Wholesale & retail | 2015 |
| Auto Correct Reading Ltd | Wholesale & retail | 2023 |
| Auto Master Marlborough Limited | Wholesale & retail | 2024 |
| Auto Motor Berkshire Ltd | Wholesale & retail | 2023 |
| Auto Remake Gurus Ltd | Wholesale & retail | 2024 |
| Auto Repair Solutions Ltd | Manufacturing | 2020 |
| Auto Skills (Reading) C.I.C. | Wholesale & retail | 2017 |
| Auto Spa Maidenhead Ltd | Administrative & support | 2025 |
| Auto Touches Limited | Wholesale & retail | 2011 |
| Autoaxis Motors Ltd | Wholesale & retail | 2023 |
| Autocare Pass Limited | Extraterritorial | 2025 |
| Autocraft (Reading) Limited | Wholesale & retail | 2007 |
| Autoforce Limited | Wholesale & retail | 2020 |
| Autofront Limited | Information & communication | 2024 |
| Autolense Ltd | Other services | 2025 |
| Automania Motors Group Ltd | Wholesale & retail | 2025 |
| Automateconnect Ltd | Administrative & support | 2024 |
| Automated Host Ltd | Wholesale & retail | 2024 |
| Automation Associates Limited | Information & communication | 2024 |
| Automation Consultants Holdings Limited | Financial services | 2020 |