| ATC Films Limited | Arts & recreation | 2016 |
| ATC Investments Limited | Real estate | 1932 |
| ATC Properties Leeds Ltd | Real estate | 2020 |
| ATC Properties Limited | Real estate | 1989 |
| ATC Trade Sales Ltd | Wholesale & retail | 2023 |
| Atcls Associate Ltd | Health & social care | 2015 |
| Atcx Limited | Wholesale & retail | 2018 |
| ATD Exports Ltd | Wholesale & retail | 2024 |
| Ateance Limited | Administrative & support | 2016 |
| Atec-Align Ltd | Wholesale & retail | 2015 |
| Ateesh Ltd | Hospitality | 2021 |
| Atelier Curated Ltd | Wholesale & retail | 2025 |
| Atelier Krier Ltd | Professional & scientific | 2024 |
| Atelier Sculpt Ltd | Other services | 2026 |
| Atex Filtration Ltd | Professional & scientific | 2020 |
| Atexpert Ltd | Education | 2022 |
| ATG Health Ltd | Health & social care | 2024 |
| ATH Online Learning Ltd | Extraterritorial | 2014 |
| Athelstan Court Limited | Construction | 2020 |
| Athena Civil Engineering Ltd | Construction | 2010 |
| Athena Dhir Trading Ltd | Other services | 2018 |
| Athena Digital Media Ltd | Professional & scientific | 2013 |
| Athena Leaders Ltd | Professional & scientific | 2019 |
| Atheris Limited | Health & social care | 2025 |
| Athill Management Company Limited | Real estate | 1993 |
| Athlete Three Zero Ltd | Professional & scientific | 2025 |
| Athletes in Schools Limited | Arts & recreation | 2024 |
| Athletic Club 2 Ltd | Arts & recreation | 2017 |
| Athletic Club Ltd | Arts & recreation | 2016 |
| Athletic Club Marketing Ltd | Arts & recreation | 2019 |
| Athletic Sportswear Ltd | Wholesale & retail | 2020 |
| Athlone Estates Ltd | Real estate | 2014 |
| Athom Healthcare Solution Limited | Health & social care | 2024 |
| Athru Coaching Ltd | Administrative & support | 2025 |
| ATI Cleaning Services Ltd | Administrative & support | 2025 |
| ATI International Ltd | Wholesale & retail | 2018 |
| Atia Property Ltd | Real estate | 2026 |
| Atin Property Management Ltd | Real estate | 2025 |
| Atiqson Enterprises Ltd | Wholesale & retail | 2025 |
| ATK Business Solutions Limited | Financial services | 2025 |
| ATK Recruitment Solutions Limited | Administrative & support | 2020 |
| Atkd Properties Limited | Real estate | 2023 |
| Atkinson Builders & Plumbers LLP | | 2012 |
| Atkinson Builders (Leeds) Ltd | Extraterritorial | 2012 |
| Atkinson Electrical (Leeds) Limited | Construction | 2018 |
| Atkinson Kassell Investments Limited | Real estate | 2016 |
| Atkinsonnotary Limited | Professional & scientific | 2012 |
| Atkinsons Fibre Solutions Ltd | Information & communication | 2024 |
| Atkinsons Property Limited | Real estate | 2023 |
| ATL Property Ltd | Financial services | 2022 |
| Atlantic C.S.P. Limited | Manufacturing | 2000 |
| Atlantic Chippy Limited | Hospitality | 2026 |
| Atlantic Commerce Solutions Limited | Wholesale & retail | 2016 |
| Atlantic Holdings Corp Ltd | Hospitality | 2023 |
| Atlantic Shelf 63 Limited | Professional & scientific | 1963 |
| Atlantic Shutters Limited | Manufacturing | 2013 |
| Atlantica Telecom Consulting Ltd | Information & communication | 2023 |
| Atlantis Forwarding Holdings Limited | Financial services | 2007 |
| Atlantis Forwarding Limited | Transport & logistics | 1977 |
| Atlas 2016 Ltd. | Construction | 2016 |
| Atlas 365 Ltd | Other services | 2023 |
| Atlas Art Services Limited | Construction | 2025 |
| Atlas Commodities Ltd | Wholesale & retail | 2014 |
| Atlas Education Consulting Limited | Professional & scientific | 2026 |
| Atlas Endoscopy Limited | Manufacturing | 2021 |
| Atlas Industrial Structures Ltd | Construction | 2021 |
| Atlas Interpreting & Translation Ltd | Professional & scientific | 2026 |
| Atlas IT Ltd | Information & communication | 2025 |
| Atlas Landers Ltd | Financial services | 2019 |
| Atlas Leisure Homes Limited | Manufacturing | 2009 |
| Atlas Medicolegal Ltd | Health & social care | 2020 |
| Atlas on SET Ltd | Hospitality | 2024 |
| Atlas Properties Yorkshire Ltd | Real estate | 2026 |
| Atlas Property Holdings Ltd | Real estate | 2017 |
| Atlas Specialist Coatings Ltd | Construction | 1999 |
| Atlis Properties Ltd | Hospitality | 2025 |
| Atlm Productions Ltd | Professional & scientific | 2019 |
| Atma Services Ltd | Health & social care | 2018 |
| ATN Marketing Limited | Wholesale & retail | 1978 |
| Atnikola Limited | Professional & scientific | 2022 |
| Atom Brewing Co Limited | Manufacturing | 2012 |
| Atom Consultants Ltd | Professional & scientific | 2012 |
| Atome Fertiliser Limited | Manufacturing | 2021 |
| Atome PLC | Financial services | 2021 |
| Atome Power Limited | Financial services | 2022 |
| Atomic Flow Limited | Professional & scientific | 2018 |
| Atomic Mind Limited | Information & communication | 2011 |
| Atomic Records Limited | Manufacturing | 2018 |
| Atomic Stacks Limited | Information & communication | 2010 |
| Atomic Waste Solutions Limited | Professional & scientific | 2024 |
| Atomic Wolf Ltd | Wholesale & retail | 2025 |
| Aton 6 Limited | Professional & scientific | 2023 |
| Atoz Saving Ltd | Wholesale & retail | 2021 |
| ATP Bathrooms Ltd | Construction | 2021 |
| ATP Plumbing Ltd | Construction | 2020 |
| ATR Property's Limited | Construction | 2014 |
| Atra Construction Ltd | Construction | 2018 |
| Atrio Homes Ltd | Real estate | 2023 |
| Atrio Search Limited | Administrative & support | 2023 |
| Atrshield Ltd | Manufacturing | 2021 |