| Apollo-Next Ltd | Information & communication | 2025 |
| Appleby Cottage C.I.C. | Arts & recreation | 2017 |
| Appleby Industries Limited | Real estate | 2023 |
| Applejam Ltd. | Professional & scientific | 2012 |
| Appleton Agency Limited | Wholesale & retail | 2024 |
| Apsurgical Ltd | Health & social care | 2024 |
| Aptus Connectivity Limited | Extraterritorial | 2014 |
| APX Construction & Management Ltd | Construction | 2025 |
| Aqua City (Hertford) Limited | Wholesale & retail | 2007 |
| Aqua Trustees Number 1 Limited | Other services | 2016 |
| Aqua Trustees Number 2 Limited | Other services | 2016 |
| Aqua-Tox Aesthetics Ltd | Other services | 2022 |
| Aquaeducta Limited | Manufacturing | 2026 |
| AR 4 Properties Ltd | Real estate | 2024 |
| AR Consortium Limited | Manufacturing | 2014 |
| AR Properties 2 Ltd | Real estate | 2017 |
| AR Properties 5 Ltd | Real estate | 2025 |
| Arbon & Miller (Lettings) Limited | Real estate | 2014 |
| Arbon & Miller Limited | Real estate | 1999 |
| Arbutus Wealth Management (AWM) Ltd | Financial services | 2010 |
| ARC Hub Studio Ltd | Professional & scientific | 2024 |
| Arcacious IP Limited | Professional & scientific | 2014 |
| Arcana Capital Limited | Information & communication | 2022 |
| Arch Aluminium and Upvc Ltd | Manufacturing | 2019 |
| Arctic FOX Events Limited | Professional & scientific | 2018 |
| Arendil Ltd | Health & social care | 2020 |
| Argall Properties Limited | Real estate | 1995 |
| Argentinian Breeders & Packers Limited | Other services | 2019 |
| ARI and DEB Limited | Wholesale & retail | 2018 |
| Ariff Ltd | Construction | 2025 |
| Aristo Management Limited | Construction | 2018 |
| Arkley Electrical Limited | Construction | 2020 |
| Arma UK Chapter | Other services | 2012 |
| Armas Advisory Limited | Professional & scientific | 2021 |
| Armie Investments Limited | Administrative & support | 2025 |
| Armr Consultancy Limited | Professional & scientific | 2023 |
| Arpl CC Limited | Construction | 2020 |
| Arshi's Aesthetics Limited | Health & social care | 2026 |
| Art and Antiques Lecture and Conference Services Limited | Administrative & support | 1995 |
| Art Explorers Limited | Education | 2018 |
| Art Vehicle Solutions Ltd | Wholesale & retail | 2017 |
| Artagnan Tech Limited | Information & communication | 2014 |
| Arthur Stevens Limited | Financial services | 2004 |
| Artificial Lawn Care Limited | Administrative & support | 2020 |
| Artshed Arts Limited | Education | 2016 |
| AS Hospitality Services Ltd | Administrative & support | 2023 |
| ASB Luxury Limited | Professional & scientific | 2014 |
| ASC Property Group Ltd | Real estate | 2019 |
| Ascension Leadership Ltd | Other services | 2023 |
| Ascensus Partners Ltd | Real estate | 2025 |
| ASH and Green Developments Ltd | Construction | 2025 |
| Ashby & Horner Holdings Limited | Administrative & support | 2000 |
| Ashby & Horner Limited | Administrative & support | 2015 |
| Ashby & Thornton Limited | Real estate | 1987 |
| Ashby Building Surveyors Limited | Professional & scientific | 2010 |
| Ashcroft Cameron (UK) Limited | Professional & scientific | 1994 |
| Ashcroft Cameron Book Keeping Limited | Professional & scientific | 1995 |
| Ashcroft Cameron Nominees Limited | Professional & scientific | 1990 |
| Ashcroft Cameron Secretaries Limited | Professional & scientific | 1991 |
| Ashdale (Hemel) Residents Company Limited | Households | 1985 |
| Ashenbury Ltd | Administrative & support | 2024 |
| Ashview Consultancy Limited | Professional & scientific | 2018 |
| Asil Automotive Ltd | Wholesale & retail | 2026 |
| Asker Estates Limited | Real estate | 2024 |
| Asker Holdings Limited | Financial services | 2023 |
| Aslan 80 Ltd | Hospitality | 2024 |
| Aspect Four Demolition Services Limited | Construction | 2009 |
| Aspire & Achieve (UK) Ltd | Health & social care | 2012 |
| Aspire Gardens Limited | Construction | 2024 |
| Asterion Building Services Limited | Wholesale & retail | 2007 |
| Asterisk Network Solutions Limited | Administrative & support | 2019 |
| Astonbury Manor (NO.1) Limited | Households | 1992 |
| Astons Residential Management Ltd | Households | 2010 |
| Astra Aquarii Productions Ltd | Information & communication | 2025 |
| Astral Home Services Limited | Construction | 2024 |
| Astral Property Investments Limited | Real estate | 2022 |
| Astral Roofing Limited | Construction | 2021 |
| Astute Management Consultants Limited | Professional & scientific | 2025 |
| AT Timber Ltd | Wholesale & retail | 2025 |
| ATA CNC Processes Limited | Wholesale & retail | 2017 |
| ATB Civils Limited | Construction | 2019 |
| ATB Plant Limited | Administrative & support | 2019 |
| Atherton Solutions Limited | Administrative & support | 2016 |
| Atlantic Breeze Services Limited | Administrative & support | 2023 |
| Atlas Facilities Services Limited | Wholesale & retail | 2013 |
| Atlas Roofing & Building Contractors Ltd | Construction | 2025 |
| Attactic Limited | Professional & scientific | 2019 |
| Attila Borzak Packaging Technology Solutions Limited | Administrative & support | 2012 |
| Attwaters Solicitors LLP | | 2024 |
| Attwaters Trust Corporation Limited | Professional & scientific | 2025 |
| AU Media Group Ltd | Professional & scientific | 2025 |
| Aubrey Lodge Management Company (Enfield) Limited | Households | 2007 |
| Audbern Investments Ltd | Real estate | 2019 |
| Audbern Limited | Financial services | 2009 |
| Auduence Ltd | Information & communication | 2025 |
| Authentic Diversity Equity & Inclusion Group Ltd | Administrative & support | 2025 |
| Autograph Group Ltd | Administrative & support | 2015 |
| Autograph Investments Ltd | Administrative & support | 2009 |
| Autograph Vehicle Solutions Ltd | Wholesale & retail | 2010 |
| Aver Consultancy Ltd | Professional & scientific | 2025 |