| Assist Psychology Consultancy Limited | Professional & scientific | 2012 |
| Assisted Business Services Ltd | Professional & scientific | 2017 |
| Assisted Care Solutions Limited | Administrative & support | 2011 |
| Associate Estates (UK) Limited | Real estate | 2016 |
| Associated Pallets Holdings Limited | Manufacturing | 2024 |
| Associated Pallets Limited | Manufacturing | 1992 |
| Assure Care Homes Ltd | Health & social care | 2014 |
| Assured Decorators Limited | Construction | 2022 |
| Assured Fleet Services Ltd | Administrative & support | 2019 |
| Assured Rider Training (Southampton) Ltd | Education | 2019 |
| Assured Travel Network Limited | Hospitality | 2025 |
| Assurize Limited | Professional & scientific | 2022 |
| Astaf Services Ltd | Manufacturing | 2013 |
| Astar Painting Ltd | Construction | 2025 |
| Astar South Solutions Limited | Other services | 2021 |
| Astareh Holdings Ltd | Financial services | 2026 |
| Asthill Limited | Households | 2007 |
| Asthma Allergy & Inflammation Research Trust Ltd | Professional & scientific | 2008 |
| Astonish Interiors Limited | Construction | 2024 |
| Astor Assets Limited | Extraterritorial | 2013 |
| Astor Energetics Ltd | Mining & quarrying | 2025 |
| Astorn Ltd | Wholesale & retail | 2018 |
| Astra SV Ltd | Transport & logistics | 2026 |
| Astra Thermal Limited | Construction | 2006 |
| Astradent (1) Limited | Health & social care | 2021 |
| Astradent (2) Limited | Health & social care | 2021 |
| Astradent Limited | Health & social care | 2020 |
| Astraea Investments Limited | Administrative & support | 2013 |
| Astraero Solutions Ltd | Professional & scientific | 2022 |
| Astrella Academy of Football Limited | Arts & recreation | 2019 |
| Astrics Electrical Services Limited | Construction | 2006 |
| Astro Air Conditioning Ltd | Construction | 2003 |
| Astrojuris Ltd | Professional & scientific | 2023 |
| Astute Carpentry & Construction Ltd | Manufacturing | 2023 |
| AT Financial Solutions Limited | Financial services | 2013 |
| At&Jw Ltd | Information & communication | 2019 |
| AT1 Advisory Limited | Administrative & support | 2024 |
| ATC Property Maintenance Limited | Construction | 2010 |
| Atco Electrical Distributors Limited | Wholesale & retail | 1999 |
| Atec Services Ltd. | Manufacturing | 2015 |
| Atech Mobile Limited | Information & communication | 2009 |
| Atelier 10 Jewellery Ltd | Manufacturing | 2012 |
| Atelier Digital Ltd | Professional & scientific | 2012 |
| Atelier Hair Boutique Limited | Other services | 2018 |
| Atelier Hair Salon Southampton Ltd | Other services | 2025 |
| Atex Global Limited | Wholesale & retail | 2009 |
| Atex Worldwide Ltd | Extraterritorial | 2020 |
| ATH Global Events Ltd | Professional & scientific | 2021 |
| Athabasca Management Limited | Professional & scientific | 2012 |
| Athem Homes Ltd | Real estate | 2019 |
| Athem Properties Trading Limited | Real estate | 2018 |
| Athena Medical Aesthetics Training Limited | Health & social care | 2019 |
| Athena Rental Properties Limited | Real estate | 2025 |
| Athena Rugs & Carpets Limited | Wholesale & retail | 2023 |
| Athenad Holdings Limited | Financial services | 2022 |
| Athenian Property Developments Ltd | Extraterritorial | 2020 |
| Athenian Property Investments Ltd | Construction | 2020 |
| Athenian Property Lease Holdings Ltd | Real estate | 2024 |
| Athenos Global Limited | Wholesale & retail | 2025 |
| Atherley Garage Limited | Wholesale & retail | 2008 |
| Atherley Recycling Limited | Water & waste | 2015 |
| Athermere OPS Limited | Information & communication | 2025 |
| Atherton Dispute Resolution Ltd | Professional & scientific | 2015 |
| Athey & Kemp (Engineers) Limited | Real estate | 1963 |
| Athlete Career Transition (ACT) Ltd | Other services | 2011 |
| Athletes & Tuxedos Ltd | Hospitality | 2022 |
| Athmonon Limited | Real estate | 2025 |
| Athwal Group Ltd | Real estate | 2025 |
| Atir Ltd | Other services | 2015 |
| Atkins Painting & Decorating Ltd | Construction | 2005 |
| ATL Consulting Ltd | Professional & scientific | 2015 |
| Atl.Cuts Ltd | Other services | 2025 |
| Atlantic Cleaners Ltd | Other services | 2017 |
| Atlantic Cleaning Centre Company Limited | Real estate | 1982 |
| Atlantic Clinic Ltd | Health & social care | 2013 |
| Atlantic Consulting Limited | Professional & scientific | 2024 |
| Atlantic Contract Furnishings Limited | Manufacturing | 2023 |
| Atlantic Investments Ltd | Professional & scientific | 2014 |
| Atlantic Propulsion Ltd | Manufacturing | 2024 |
| Atlantic Yachts SW Limited | Wholesale & retail | 2018 |
| Atlantis Marine Steel Limited | Construction | 2022 |
| Atlas Building Plastics Ltd | Wholesale & retail | 2024 |
| Atlas Electrical Projects Ltd | Construction | 2024 |
| Atlas Global Synergy Limited | Professional & scientific | 2018 |
| Atlas Import Solutions Limited | Wholesale & retail | 2025 |
| Atlas Leisure Limited | Arts & recreation | 2006 |
| Atlas Marketing & Recruitment Ltd | Professional & scientific | 2020 |
| Atlas Ortho Ltd | Health & social care | 2025 |
| Atlas OZ Ltd | Transport & logistics | 2026 |
| Atlas South Construction Ltd | Construction | 2024 |
| Atom Brothers Ltd | Administrative & support | 2023 |
| Atom Healthcare Ltd | Wholesale & retail | 2022 |
| Atom IP Ltd | Information & communication | 2016 |
| Atom Network Limited | Information & communication | 2017 |
| Atomic Marketing Co Ltd | Information & communication | 2012 |
| Atomic Tangerine Ltd | Arts & recreation | 2015 |
| Atomz Property Maintenance Limited | Transport & logistics | 2023 |
| ATR Consulting Limited | Administrative & support | 2006 |
| ATR Infotech Limited | Information & communication | 2026 |
| Atree Limited | Hospitality | 2021 |