| Amplified Consultancy Limited | Information & communication | 2018 |
| Ampthill Grange Ltd | Extraterritorial | 2011 |
| AMY & Ross Consulting Ltd | Professional & scientific | 2025 |
| Anamsa Real Estate Limited | Extraterritorial | 2024 |
| Anatolian Cuisine Ltd | Hospitality | 2022 |
| Ancaster Gate Management Limited | Real estate | 2017 |
| AND More Media Ltd | Information & communication | 2022 |
| Andersonrisk Limited | Professional & scientific | 2015 |
| Andiamo Business Limited | Extraterritorial | 2021 |
| Andrena Limited | Professional & scientific | 2003 |
| Andrewsfield Limited | Households | 2006 |
| Androfertility Limited | Financial services | 2016 |
| Android Epos Ltd | Professional & scientific | 2019 |
| Anfora Properties Ltd | Construction | 2019 |
| Anglia Accounting Limited | Professional & scientific | 2019 |
| Anglia Consulting Limited | Professional & scientific | 2019 |
| Angry Donkey Hidden Dodo Limited | Real estate | 2021 |
| ANJ Trading Limited | Manufacturing | 2003 |
| Anna Brennan Events Ltd | Professional & scientific | 2015 |
| Annables (Management) Company Limited | Households | 1960 |
| Anova Holdings Limited | Financial services | 2021 |
| Anstone Properties Limited | Real estate | 2021 |
| Anterprise Group Ltd | Real estate | 2024 |
| Anthony Better Solutions Limited | Professional & scientific | 2026 |
| AP Alphadeck Ltd | Administrative & support | 2020 |
| APA Hr&Consulting&Software Ltd | Information & communication | 2021 |
| Apartamento Marina Limited | Real estate | 2020 |
| APN Financial Planning Ltd | Professional & scientific | 2020 |
| Apod PR and Communications Limited | Professional & scientific | 2016 |
| Apreslabs Ltd | Professional & scientific | 2011 |
| Aquamarine Marketing Limited | Professional & scientific | 2011 |
| Arabian BEC Limited | Professional & scientific | 2013 |
| Aran Freehold Limited | Real estate | 2015 |
| Arancini Solutions Ltd | Professional & scientific | 2025 |
| Arbenneck Developments Limited | Construction | 2021 |
| Archco Assets Limited | Wholesale & retail | 2023 |
| Archimad Limited | Wholesale & retail | 2019 |
| Archway Property Investment Limited | Real estate | 2018 |
| Arcs Solutions Limited | Administrative & support | 2019 |
| ARE Advisors Limited | Professional & scientific | 2023 |
| Areal IT Solutions Ltd | Information & communication | 2014 |
| Argonaut Business Intelligence Limited | Information & communication | 2015 |
| ARH Marketing and Consulting Ltd | Professional & scientific | 2019 |
| Ariba Street Ltd | Hospitality | 2025 |
| ARK Property Management Limited | Real estate | 2013 |
| Arkticc Technologies Ltd | Information & communication | 2021 |
| Arnika Ltd | Information & communication | 2014 |
| Around City Ltd | Real estate | 2023 |
| Arrow Innovation Limited | Professional & scientific | 2013 |
| Art Deco Apartments Rugby Ltd | Households | 2014 |
| Art of Estates Limited | Other services | 2023 |
| Art of Trading Ltd. | Other services | 2015 |
| Art, Science and Magic Limited | Information & communication | 2020 |
| Artemis Nannies Limited | Other services | 2022 |
| Artofus Limited | Information & communication | 2016 |
| Artroomco Limited | Education | 2024 |
| ASH Properties London Ltd | Real estate | 2021 |
| Ashberry Care Homes Limited | Health & social care | 2025 |
| Ashberry Care Services Limited | Health & social care | 2017 |
| Ashberry Healthcare Limited | Health & social care | 2003 |
| Ashberry Holdings Limited | Health & social care | 2019 |
| Ashford Interiors Limited | | 1986 |
| Ashton Crescent Residents Association Limited | Other services | 2002 |
| ASL Consulting Services Ltd | Information & communication | 2025 |
| Aspire Radiology Ltd | Health & social care | 2020 |
| Aspyr Media Europe Limited | Information & communication | 2004 |
| Asset Resourcing Limited | Administrative & support | 2005 |
| Assetware Limited | Information & communication | 1987 |
| Assl Consultants Limited | Administrative & support | 2015 |
| Aston Media Limited | Professional & scientific | 2021 |
| Astra Heed Ltd | Information & communication | 2024 |
| Astraia Limited | Professional & scientific | 2012 |
| Asurg Limited | Health & social care | 2025 |
| Atelic UX Research Ltd | Wholesale & retail | 2019 |
| Atlas Systems Engineering Ltd | Professional & scientific | 2024 |
| Atlas Yachting Limited | Manufacturing | 2021 |
| Audiobook Narration UK Limited | Arts & recreation | 2026 |
| Augustus Enterprise Ltd | Real estate | 2024 |
| Aura-Amoura Limited | Extraterritorial | 2015 |
| Auralis Technologies Ltd | Information & communication | 2026 |
| Auroch Capital Advisory Ltd | Professional & scientific | 2021 |
| Auron Holdings Ltd | Financial services | 2025 |
| Austele Services Ltd | Other services | 2012 |
| Automatic Songs Limited | Arts & recreation | 1996 |
| Automotive Professional Services Limited | Professional & scientific | 2005 |
| Av-Zone Limited | Construction | 2003 |
| Avalon Management Consultants Ltd | Administrative & support | 2019 |
| Avon Court Harpenden Limited | Real estate | 1997 |
| Awesome Engineers Harpenden Ltd | Education | 2016 |
| Axle Consulting Limited | Professional & scientific | 2020 |
| Ayazgok Ltd | Health & social care | 2019 |
| Ayemcnath Associates Limited | Administrative & support | 2009 |
| Ayres END Limited | Agriculture & fishing | 2025 |
| Ayrton Lawrence Ltd | Construction | 2024 |
| Azuna Solutions Ltd | Administrative & support | 2023 |
| B & B Timber & Builders Merchants Limited | Wholesale & retail | 2010 |
| B Cubed Limited | Professional & scientific | 2025 |
| B Twelve Limited | Information & communication | 2025 |
| B&N Building and Electrical Limited | Construction | 2019 |
| B.M.Hilton & Company Limited | Construction | 1971 |