| Ambergate Developments (MK) Limited | Construction | 1994 |
| Ambrit Limited | Information & communication | 1984 |
| AMD Construct Group Ltd | Construction | 2024 |
| AMD Pubs Limited | Hospitality | 2015 |
| Amdeus Expertise Limited | Professional & scientific | 2023 |
| Amed 49 Ltd | Other services | 2025 |
| Amely Properties Limited | Real estate | 2023 |
| Amenbury (Watermills) Limited | Professional & scientific | 2020 |
| Amenbury Court Residents Association (Harpenden) Limited | Households | 1966 |
| Amenbury Property Limited | Other services | 2003 |
| Amic Global Services Limited | Wholesale & retail | 2022 |
| Amiju Property Ltd | Real estate | 2019 |
| Amina Cura Ltd | Other services | 2024 |
| AMK Luxury Limited | Wholesale & retail | 2025 |
| AMM VAS Limited | Administrative & support | 2019 |
| Ammt Consultancy Ltd | Households | 2022 |
| Amnoki Limited | Construction | 2021 |
| Ampras Realty Ltd | Real estate | 2025 |
| Ampton Limited | Hospitality | 2020 |
| Amro Catering & Events Limited | Hospitality | 2021 |
| Amro Estate Holdings Limited | Real estate | 2025 |
| Amzam Holdco Limited | Financial services | 2024 |
| Anacons Ltd. | Health & social care | 2015 |
| Anadav Ltd | Construction | 2015 |
| Anais Logistics Ltd | Transport & logistics | 2025 |
| Analytical Laboratory Informatics Solutions Limited | Administrative & support | 2006 |
| Anaya Transport Ltd | Transport & logistics | 2025 |
| ANC Groundworks Ltd | Construction | 2022 |
| Anderson Elevator Services Ltd | Construction | 2010 |
| Anderson Waste Management Ltd | Water & waste | 2021 |
| Andersons Croft Management Company Limited | Households | 2012 |
| Andreea Clinic Limited | Construction | 2024 |
| Andrew Express Haulage Ltd | Transport & logistics | 2025 |
| Andrews Electrical Contractors Limited | Construction | 2004 |
| Andrews Johnston Ltd | Construction | 2021 |
| ANE Fuel & GAS Ltd | Wholesale & retail | 2019 |
| Anedy 4 Future Limited | Transport & logistics | 2023 |
| ANF Service Limited | Wholesale & retail | 2019 |
| Angel Concepts Ltd | Wholesale & retail | 2012 |
| Angela Electronics Limited | Wholesale & retail | 2019 |
| Angelo Transport 2024 Ltd | Transport & logistics | 2024 |
| Angki Enterprises Ltd | Wholesale & retail | 2020 |
| Anglian Tree Services Ltd | Agriculture & fishing | 2017 |
| ANH Associates Limited | Professional & scientific | 2016 |
| Animo Estates Ltd | Real estate | 2019 |
| Aniqah Hygiene Ltd | Health & social care | 2016 |
| Anjoman Fresh Finds Ltd | Wholesale & retail | 2025 |
| Anna Byrsan Ltd | Construction | 2024 |
| Anno Domino Building Properties Limited | Real estate | 2025 |
| Anno Domino Properties Limited | Real estate | 2025 |
| Anodamine UK Ltd | Manufacturing | 2026 |
| Anon (Chesterfield) Ltd | Construction | 2019 |
| Anstey & Lamond Ltd | Construction | 2026 |
| Ante Transport Limited | Transport & logistics | 2022 |
| Antevorta Consultants Limited | Professional & scientific | 2013 |
| Anthony Shevlin Construction Ltd | Construction | 2018 |
| Anthope Care Ltd | Public administration | 2022 |
| Antilles Network Limited | Other services | 2025 |
| Antjan Limited | Transport & logistics | 2019 |
| Anton's Groundworks Limited | Construction | 2023 |
| Aolta Communications Ltd | Construction | 2022 |
| AP Car Services Ltd | Wholesale & retail | 2023 |
| AP Dairies Limited | Wholesale & retail | 2013 |
| AP Global Traders Ltd | Wholesale & retail | 2025 |
| AP SD Forty Three Limited | Wholesale & retail | 2025 |
| APETREI26 Ltd | Transport & logistics | 2023 |
| Apex Fades Ltd | Other services | 2025 |
| Apex Fleet Services Limited | Wholesale & retail | 2026 |
| Apexguard Roofing & Building Ltd | Construction | 2026 |
| APL Quality Services Ltd | Wholesale & retail | 2025 |
| APO Construction Ltd | Construction | 2022 |
| Apollo Energy Group Ltd | Energy | 2025 |
| Apollo Global Research and Consultancy Limited | Professional & scientific | 2021 |
| Appleyard (Harpenden) Management Company Limited | Households | 2019 |
| Apresol LLP | | 2016 |
| April George Services Limited | Transport & logistics | 2014 |
| APT Contracting Limited | Professional & scientific | 2024 |
| APTISH-66 Ltd | Transport & logistics | 2020 |
| Aptsurvey Limited | Professional & scientific | 2024 |
| APW Roofing Ltd | Construction | 2022 |
| Aqua Business Consultants Limited | Professional & scientific | 2014 |
| Aquatox Health Limited | Other services | 2021 |
| Arabian Code Limited | Wholesale & retail | 2025 |
| Arastee Ltd | Hospitality | 2023 |
| Arayish Limited | Wholesale & retail | 2016 |
| Arbour Landscape Solutions Ltd | Construction | 2019 |
| ARC SS Ltd | Financial services | 2023 |
| Arca Nautica Ltd | Wholesale & retail | 2015 |
| Arca Tours & Travel Ltd | Transport & logistics | 2018 |
| Arcada One Ltd | Professional & scientific | 2024 |
| Archem PRS Limited | Manufacturing | 2009 |
| Architecture Studio 5 Ltd | Professional & scientific | 2015 |
| ARCHITOUR360 Ltd | Professional & scientific | 2025 |
| Arcovia Services Limited | Administrative & support | 2026 |
| Arden Grove Management Limited | Real estate | 2016 |
| Arena Airport Transfers Ltd | Transport & logistics | 2024 |
| Argyll AI Limited | Professional & scientific | 2026 |
| Aristocuts Limited | Other services | 2004 |
| Arjd Brickworks Ltd | Construction | 2025 |
| Arje Productions Limited | Information & communication | 2026 |