| Assetsky Limited | Construction | 1988 |
| Assimila Group Limited | Financial services | 2019 |
| Assimila Limited | Professional & scientific | 2006 |
| Assiral Limited | Construction | 2017 |
| Assist Mortgages Ltd | Financial services | 2025 |
| Assisting Angels Care Ltd | Health & social care | 2024 |
| Associated County Surfacing Limited | | 1974 |
| Association of Artificial Intelligence | Public administration | 2026 |
| Association of Dog Boarders | Other services | 2019 |
| Association of Facilitators | Other services | 2011 |
| Assur Holdings Ltd | Financial services | 2026 |
| Assur Limited | Information & communication | 2011 |
| Assure Investments Limited | Real estate | 2015 |
| Assure Techsol Ltd | Information & communication | 2026 |
| Assured Accounting Limited | Professional & scientific | 2016 |
| Assured Customs Declarations Ltd | Transport & logistics | 2020 |
| Assured Interior Solutions Ltd | Construction | 2002 |
| Assured Training Support Ltd | Public administration | 2024 |
| Assuria Limited | Information & communication | 2004 |
| AST (London Court) Limited | Real estate | 2021 |
| AST Management Limited | Information & communication | 2006 |
| AST Reading Ltd | Construction | 2019 |
| Astafor Limited | Wholesale & retail | 2019 |
| Astana Property Limited | Real estate | 2022 |
| Astella Technologies UK Limited | Information & communication | 2024 |
| Astelware Ltd | Information & communication | 2014 |
| Aster Garden Design Ltd | Professional & scientific | 2025 |
| Asterix Management Ltd | Real estate | 2023 |
| Asthetia Ltd | Wholesale & retail | 2012 |
| Astinal Consulting Limited | Administrative & support | 2016 |
| Astir Capital Advisors Limited | Financial services | 2009 |
| Aston Care Limited | Health & social care | 2006 |
| Aston Carter Tracks Ltd | Information & communication | 2017 |
| Aston Court (Earley) Property Management Limited | Households | 2015 |
| Aston Fisher Limited | Administrative & support | 2013 |
| Aston Gate Management Company Limited | Extraterritorial | 2018 |
| Aston Zoraster International Search Ltd | Administrative & support | 2024 |
| Astonish AI Ltd | Information & communication | 2026 |
| Astounding Care Services Ltd | Health & social care | 2020 |
| Astra Infotech Limited | Information & communication | 2001 |
| Astrass Consulting Ltd | Professional & scientific | 2011 |
| Astratus Limited | Manufacturing | 2024 |
| Astrum Private Ltd | Hospitality | 2013 |
| Asusual Consultants Ltd | Health & social care | 2015 |
| ASW Recruitment Consultants Limited | Administrative & support | 2019 |
| Aszlan Properties Ltd | Real estate | 2025 |
| AT Hand Supported Living Services Limited | Health & social care | 2020 |
| AT Innovation Limited | Professional & scientific | 2022 |
| AT Logistics Ltd | Wholesale & retail | 2021 |
| AT WAR With Myself Ltd | Wholesale & retail | 2019 |
| AT Your Peak Ltd | Health & social care | 2018 |
| ATA Assistance Limited | Administrative & support | 2025 |
| Atal Foods Ltd | Hospitality | 2019 |
| Atalta Ltd | Extraterritorial | 2015 |
| Atancio Limited | Transport & logistics | 2025 |
| Atasseri Ltd | Information & communication | 2019 |
| ATB Accounting Ltd | Professional & scientific | 2015 |
| Ates Capital Holdings Limited | Real estate | 2023 |
| Atforrest Ltd | Professional & scientific | 2019 |
| ATG Broadcast Limited | Extraterritorial | 2014 |
| ATG Spectrum (UK) Limited | Extraterritorial | 2001 |
| Athena Associates Ltd | Professional & scientific | 2022 |
| Athena's Daughters Limited | Households | 2010 |
| Athenafashion Trading Limited | Wholesale & retail | 2012 |
| Athenas Gold Ltd | Wholesale & retail | 2025 |
| Athletico Seville Fitness Ltd | Arts & recreation | 2025 |
| Athos Marketing Ltd | Wholesale & retail | 2022 |
| Athos Mediation Limited | Professional & scientific | 2022 |
| Athos Property Deal Ltd | Real estate | 2023 |
| Athos Property Investment Ltd | Real estate | 2023 |
| Athos Property Ltd | Real estate | 2023 |
| Athos UK Property Relocation Ltd | Hospitality | 2023 |
| Atilaksmi Technologies Ltd | Information & communication | 2014 |
| ATJ3 Adventures Ltd | Wholesale & retail | 2019 |
| ATK Reading Limited | Real estate | 2025 |
| Atkins Martin Associates Limited | Professional & scientific | 1976 |
| Atkins Martin Consultants Limited | Information & communication | 1989 |
| Atlantic Green UK Limited | Energy | 2021 |
| Atlantic Travel Group Limited | Transport & logistics | 2020 |
| Atlantis Building Supplies Ltd | Wholesale & retail | 2022 |
| Atlantis Games Limited | Information & communication | 2015 |
| Atlantis Web Marketing Limited | Information & communication | 2013 |
| Atlantix UK Direct Ltd | Wholesale & retail | 2026 |
| Atlas Drive Solutions Ltd | Transport & logistics | 2026 |
| Atlas Geomatics Ltd | Professional & scientific | 2019 |
| Atlas Infrastructure Partners (UK) Ltd | Financial services | 2017 |
| Atlas Merchant Capital Holdings UK Limited | Administrative & support | 2014 |
| Atlas Merchant Capital UK LLP | | 2016 |
| Atlas Merchants Supply Co Ltd | Wholesale & retail | 2026 |
| Atlas Pensions Consultancy Services Limited | Financial services | 2014 |
| Atlas Property NE Ltd | Real estate | 2023 |
| Atlas Shawarma Limited | Hospitality | 2026 |
| Atlico Limited | Real estate | 2017 |
| ATM Cashpoint Group Limited | Wholesale & retail | 2010 |
| ATM Retail Ltd | Wholesale & retail | 2025 |
| Atmosphere 9 Ltd | Professional & scientific | 2017 |
| Atom BIN Cleaning Services LLP | | 2026 |
| Atom Global Saas Ltd | Extraterritorial | 2024 |
| ATOM2 Technologies Limited | Information & communication | 2024 |
| Atomic Increment Ltd | Information & communication | 2021 |