| Armstrong Property Maintenance Ltd | Administrative & support | 2002 |
| Armstrong Young Dental Equipment LLP | | 2016 |
| Arpin International (UK) Limited | Administrative & support | 2003 |
| Arpin International Group Limited | Administrative & support | 2014 |
| Arrowcroft Group Ltd | Real estate | 2021 |
| Art Publishing Inc Limited | Information & communication | 2023 |
| Artem Winner Ltd | Construction | 2018 |
| Arthur Enterprises Limited | Real estate | 2016 |
| Arthur&Olive Ltd | Wholesale & retail | 2022 |
| Artificial Intelligence Strategies Ltd | Information & communication | 2024 |
| Artistic Ventures Limited | Wholesale & retail | 2022 |
| Artivane Ltd | Wholesale & retail | 2025 |
| Artman Build Ltd | Construction | 2018 |
| AS Cardiac Care Ltd | Health & social care | 2019 |
| AS Gardens Ltd | Administrative & support | 2021 |
| Asar Consultancy Ltd | Information & communication | 2023 |
| ASB Partners LLP | | 2007 |
| Asbury Container Services Limited | Transport & logistics | 2014 |
| Ascentmed Ltd | Education | 2025 |
| Asenta Capital Limited | Extraterritorial | 2012 |
| ASH Technical Services Limited | Energy | 2004 |
| ASHAD225 Investments Limited | Real estate | 2025 |
| Ashbyrne Homes (Fleet House) Limited | Construction | 2022 |
| Ashbyrne Homes Ltd | Construction | 2016 |
| Ashdown Finance 2025 Limited | Professional & scientific | 2025 |
| Ashdown Finance Limited | Professional & scientific | 1994 |
| Ashgrove Bespoke Cabinetry Ltd | Manufacturing | 2025 |
| Ashgrove Construction Limited | Construction | 2025 |
| Ashley Consultancy LLP | | 2017 |
| Ashleys Contractors Limited | Construction | 2015 |
| Ashleys Holdings Ltd | Financial services | 2024 |
| Ashmore Nicholson Group Ltd | Administrative & support | 2023 |
| Ashmour Ltd | Professional & scientific | 2026 |
| Ashwold Investments Limited | Professional & scientific | 2022 |
| Asianbeautywholesale UK Limited | Wholesale & retail | 2023 |
| Askant IT Limited | Information & communication | 2013 |
| ASM Consulting Partnership LLP | | 2023 |
| Aspects Together CIC | Health & social care | 2023 |
| Aspicorn Limited | Financial services | 2007 |
| Aspiring Angels Ltd | Wholesale & retail | 2022 |
| ASR Medicolegal Ltd | Health & social care | 2026 |
| Asra Care Specialist Ltd | Health & social care | 2024 |
| Asset Professional Services Limited | Professional & scientific | 2001 |
| Association for Animals Horticultural & Equine Practitioners CIC | Agriculture & fishing | 2019 |
| Assured Roofing and Construction Limited | Construction | 2017 |
| Asta Capital Partners Services (UK) Limited | Administrative & support | 2025 |
| Asta Investment Advisors Ltd | Financial services | 2019 |
| Astor Homes Limited | Construction | 2010 |
| Asunsti Limited | Wholesale & retail | 2022 |
| Asylum Speakers Ltd | Education | 2024 |
| AT the Dog Groomers Ltd | Other services | 2021 |
| ATC Goods & Services Limited | Professional & scientific | 2024 |
| Atelier 98 Limited | Real estate | 2004 |
| Atelier Alexandra Ltd | Professional & scientific | 2024 |
| Atelier Blue Limited | Real estate | 2002 |
| Atelier Olivier Ltd | Manufacturing | 2026 |
| Atelier Rouge Limited | Real estate | 2003 |
| Atendco Ltd | Manufacturing | 2012 |
| Aterio Family Support Ltd | Health & social care | 2021 |
| Atholview Limited | Real estate | 1983 |
| Atlas Coaching Ltd | Arts & recreation | 2025 |
| Atlas Containers Ltd | Construction | 2020 |
| Atlas Helix Co. Ltd | Financial services | 2025 |
| Atlaseco Ltd | Arts & recreation | 2021 |
| Atom Brands Limited | Manufacturing | 2018 |
| Atom Cask Holdings Limited | Manufacturing | 2018 |
| Atom Drinks Limited | Manufacturing | 2018 |
| Atom Group Limited | Manufacturing | 2018 |
| Atom Supplies Limited | Manufacturing | 1996 |
| ATR London Ltd | Wholesale & retail | 2019 |
| Atrium LDN LLP | | 2013 |
| Atrium Ventures Ltd | Construction | 2012 |
| Atta Associates Ltd | Information & communication | 2020 |
| Atta Capital Ltd | Real estate | 2024 |
| Attention Seekers Marketing Limited | Professional & scientific | 2023 |
| Audiophiles Clinic Ltd | Professional & scientific | 2014 |
| Aujla Investments Limited | Financial services | 2013 |
| Aura Hair Design Limited | Other services | 2017 |
| Aurora - Garden of Soul Ltd | Health & social care | 2023 |
| Aurora Paediatric Endocrine Services Ltd | Health & social care | 2023 |
| Aurvandill Kennel & Livery Limited | Agriculture & fishing | 2025 |
| Auster Court Management Company Ltd | Households | 2020 |
| Auto 500 Ltd | Wholesale & retail | 2020 |
| Auto Enlighten Ltd | Wholesale & retail | 2015 |
| Auto Revival Ltd | Wholesale & retail | 2025 |
| Auto-Sportiva Limited | Wholesale & retail | 2008 |
| Autolink International Limited | Wholesale & retail | 1997 |
| Automated Lighting Limited | Administrative & support | 2009 |
| Autonomous Flight Ltd | Administrative & support | 2017 |
| Autoright (Tonbridge) Ltd | Wholesale & retail | 2022 |
| Autoright Ltd | Wholesale & retail | 2016 |
| AVB Business Solutions Ltd | Professional & scientific | 2014 |
| Avenel Investments Limited | Information & communication | 2019 |
| Avenley Engineering Limited | Manufacturing | 2012 |
| Avery ROW Capital Holdings Limited | Financial services | 2023 |
| Avery ROW Capital Partners LLP | | 2016 |
| Avery ROW Capital Services (UK) Ltd | Other services | 2016 |
| Aviemore Building Contractors Limited | Construction | 1999 |
| Aviso Consultancy Ltd | Professional & scientific | 2012 |
| AVO Solar Limited | Construction | 2011 |