| ATIBHAI.1 Limited | Other services | 2018 |
| Atize Limited | Real estate | 2019 |
| ATJ Country Escapes Ltd | Real estate | 2022 |
| ATK Digital Business Ltd | Wholesale & retail | 2025 |
| Atkin Trade Framing Limited | Construction | 2025 |
| Atkin Trade Specialists Limited | Construction | 1978 |
| Atkinson Europe PLC | Administrative & support | 2019 |
| Atkinson's Properties Limited | Real estate | 2012 |
| Atlanta Commercial Property Management Limited | Real estate | 2025 |
| Atlanta Enterprises Limited | Financial services | 2023 |
| Atlantic Auto Salvage Ltd | Wholesale & retail | 2018 |
| Atlantic Building Services Limited | Real estate | 1996 |
| Atlantic Business PC | | 2017 |
| Atlantic Cafe Cardiff Ltd | Hospitality | 2022 |
| Atlantic Cladding Limited | Wholesale & retail | 1992 |
| Atlantic Construction Builders Ltd | Construction | 2014 |
| Atlantic Dwellings Limited | Construction | 2006 |
| Atlantic Ecopark Limited | Water & waste | 2006 |
| Atlantic Food Services Ltd | Wholesale & retail | 2022 |
| Atlantic General Trading Ltd | Wholesale & retail | 2023 |
| Atlantic Management Ltd | Financial services | 2020 |
| Atlantic Media Group LLP | | 2025 |
| Atlantic Pacific Marine Limited | Construction | 2010 |
| Atlantic Polymers Limited | Manufacturing | 1994 |
| Atlantic Power Limited | Water & waste | 2006 |
| Atlantic Printing Ltd | Manufacturing | 2024 |
| Atlantic Property Developments PLC | Real estate | 1989 |
| Atlantic Property Services (Cardiff) Limited | Real estate | 2004 |
| Atlantic Property Swansea Ltd. | Real estate | 2016 |
| Atlantic Recycling Limited | Water & waste | 2006 |
| Atlantic Resource (Services) Limited | Administrative & support | 2013 |
| Atlantic Ventilation Limited | Construction | 2011 |
| Atlantic Venture Capital Limited | Real estate | 1993 |
| Atlantic Waste Management Limited | Water & waste | 2006 |
| Atlantica Plumbing and Heating Ltd | Construction | 2021 |
| Atlantis Academy Ltd | Education | 2014 |
| Atlantis Catering Limited | Hospitality | 2019 |
| Atlantis Consultancy & Training Ltd | Professional & scientific | 2010 |
| Atlantis Elements Limited | Information & communication | 2019 |
| Atlantis Holdings Limited | Real estate | 2021 |
| Atlantis Investments (UK) Ltd | Real estate | 2015 |
| Atlantis World Ltd | Professional & scientific | 2019 |
| Atlantya Ltd | Professional & scientific | 2016 |
| Atlas Artisnal Ltd | Other services | 2024 |
| Atlas Civils Ltd | Real estate | 2023 |
| Atlas Contracting Ltd | Construction | 2023 |
| Atlas Cosmetics Clinic Ltd | Health & social care | 2021 |
| Atlas Digital Soft Ltd | Information & communication | 2025 |
| Atlas Employment Temps Ltd | Administrative & support | 2020 |
| Atlas Fashion (UK) Limited | Wholesale & retail | 2014 |
| Atlas Globus Ltd | Professional & scientific | 2025 |
| Atlas Grove Enterprises Ltd | Households | 2026 |
| Atlas Hill Ltd | Financial services | 2012 |
| Atlas Independent Financial Services Limited | Financial services | 2013 |
| Atlas Investments Group Limited | Construction | 2026 |
| Atlas Plant Services Ltd | Administrative & support | 2026 |
| Atlas Precision Welding Ltd. | Manufacturing | 2020 |
| Atlas Profit World Ltd | Information & communication | 2017 |
| Atlas Property Ventures Limited | Households | 2024 |
| Atlas Resources Footprint AI Limited | Financial services | 2024 |
| Atlas Soft DEV Ltd | Information & communication | 2025 |
| Atlas Sporting Ltd | Administrative & support | 2019 |
| Atlas Summit Limited | Wholesale & retail | 2024 |
| Atlas Sure Ltd | Wholesale & retail | 2026 |
| Atlas Tyre Service (Cardiff) Limited | Wholesale & retail | 2017 |
| Atlas UK Global Access Limited | Hospitality | 1988 |
| Atlaschic Ltd | Wholesale & retail | 2025 |
| Atlasdutoit Ltd | Construction | 2010 |
| Atlus Electrical Contractors Limited | Construction | 2013 |
| ATM Post Limited | Real estate | 2015 |
| Atms Cyber Securities UK Ltd | Information & communication | 2013 |
| ATO Marketing Ltd | Wholesale & retail | 2018 |
| Atock Homes and Care Services Ltd | Public administration | 2018 |
| Atom Consultancy Solutions Ltd | Administrative & support | 2014 |
| Atom Experience Design Ltd | Professional & scientific | 2016 |
| Atom Household Group Limited | Manufacturing | 2025 |
| ATOM360 CYF | Information & communication | 2026 |
| Atomas Tech Ltd | Information & communication | 2026 |
| Atomic North Technologies Ltd | Wholesale & retail | 2024 |
| Atomic Smart Ltd | Construction | 2020 |
| Atomicsolutions Ltd | Information & communication | 2021 |
| Atominc UK Limited | Wholesale & retail | 2022 |
| Atosh Limited | Information & communication | 2019 |
| Atoz Food Limited | Hospitality | 2026 |
| Atplay Marketing Ltd | Professional & scientific | 2024 |
| ATR Electronics Ltd | Information & communication | 2019 |
| Atrack Soon SL Limited | Wholesale & retail | 2025 |
| Atradius Collections Limited | Administrative & support | 1997 |
| Atradius Pension Trustees Limited | Financial services | 1991 |
| Atrium Alliance PC | | 2017 |
| Atsli Casting Limited | Arts & recreation | 2003 |
| Atsys Ltd | Construction | 2015 |
| Attain & Sustain Health & Fitness Limited | Arts & recreation | 2017 |
| Attard's Ltd | Other services | 2013 |
| Attic Asset Ltd | Wholesale & retail | 2021 |
| Attic Inventory Limited | Professional & scientific | 2011 |
| Atticus Cafe Bar & Grill Ltd | Hospitality | 2019 |
| Atticus Cleaning Services Limited | Administrative & support | 2016 |
| Attique.Co.Uk Limited | Construction | 2024 |
| Attius Consultancy Ltd | Professional & scientific | 2020 |