| Aryan's Physiotherapy Ltd | Information & communication | 2015 |
| AS Contractors Ltd | Construction | 2016 |
| AS Group & Landscaping Ltd | Construction | 2025 |
| AS Homes and Construction UK Limited | Construction | 2018 |
| AS Laundries Limited | Other services | 2025 |
| Ascencia Fire Consulting Limited | Manufacturing | 2011 |
| Ascensa Consultants Limited | Professional & scientific | 2013 |
| Ascension Ground Productions Ltd | Information & communication | 2025 |
| ASE Logistics Ltd | Transport & logistics | 2025 |
| ASE Productions Ltd | Arts & recreation | 2019 |
| Aseea Limited | Hospitality | 2024 |
| ASH Tree Grove Management Ltd | Households | 2024 |
| Ashby Finance Recruitment Ltd | Administrative & support | 2008 |
| Ashdown Gardens (Eridge Road) Residents Management Company Limited | Households | 2021 |
| Ashley Consulting Limited | Professional & scientific | 2006 |
| Asho Group Ltd | Construction | 2024 |
| Ashridge Building and Construction Limited | Construction | 2023 |
| Ashridge Grange (Wokingham) Management Company Limited | Households | 2021 |
| Ashridge Mechanical & Electrical Limited | Construction | 2016 |
| Ashridge Property Developments Limited | Construction | 2020 |
| Ashridge Solutions Limited | Professional & scientific | 2025 |
| Ashtree HR Limited | Administrative & support | 2012 |
| Asif Naseem Healthcare Ltd | Health & social care | 2022 |
| Askim Limited | Professional & scientific | 2015 |
| Aslan Associates Limited | Administrative & support | 2007 |
| Aslan Life Ministries Ltd | Other services | 2018 |
| ASM Construction Ltd | Construction | 2019 |
| Asmark Ltd | Financial services | 2023 |
| Aspect Flooring (Holdings) Limited | Construction | 2020 |
| Aspect Flooring Limited | Construction | 2009 |
| Aspen Animal Health Ltd | Manufacturing | 2016 |
| Aspen Walk (Eight ASH Green) Management Company Limited | Households | 2020 |
| Aspens Place Management Company Limited | Real estate | 2005 |
| Aspire Academies Trust | Education | 2012 |
| Aspire Executive Cars Ltd | Transport & logistics | 2023 |
| Aspire Plant Hire Ltd | Administrative & support | 2026 |
| Aspire Property & Refurbishments Ltd | Construction | 2024 |
| Aspire Technology Consulting Limited | Information & communication | 2012 |
| Aspiria Recruit Limited | Administrative & support | 2017 |
| Asprey Group Ltd | Administrative & support | 2015 |
| Asra Analytics Limited | Information & communication | 2024 |
| Asra Care Group Ltd | Health & social care | 2025 |
| Asset Construction UK Limited | Extraterritorial | 2015 |
| Asset Iris Ltd | Information & communication | 2026 |
| Asset Mechanical Services Limited | Manufacturing | 2013 |
| Assiatu Limited | Health & social care | 2019 |
| Assimilate Consulting Ltd | Professional & scientific | 2019 |
| Associates Roofing Limited | Construction | 2025 |
| Association of British Schools Overseas | Education | 2019 |
| Association of Certified Accountants Ltd | Professional & scientific | 2026 |
| Association of Public Certified Accountants | Education | 2022 |
| AST Structural Engineering Services Limited | Professional & scientific | 2002 |
| Aston Pharmaceuticals Ltd | Manufacturing | 2019 |
| Aston View and Elstree Road Flat Management Company Limited | Households | 2009 |
| Astra Lumiere Ltd | Wholesale & retail | 2025 |
| Astral Arts Limited | Information & communication | 2010 |
| Astral Enterprises Ltd | Information & communication | 2023 |
| Astril Limited | Other services | 1979 |
| Astwick Manor Management Company Limited | Households | 2004 |
| Astwood Construction Limited | Construction | 2022 |
| Astwood Securities Limited | Real estate | 1999 |
| Asus Care UK Limited | Health & social care | 2018 |
| ASZ Delivery Solution Ltd | Transport & logistics | 2022 |
| AT Hestia Limited | Wholesale & retail | 2026 |
| AT Properties 4 YOU Limited | Real estate | 2024 |
| Atakamite Limited | Manufacturing | 2018 |
| Atao Tech Ltd | Information & communication | 2025 |
| ATD Trade & Holding Ltd | Financial services | 2025 |
| ATF 16 Ltd | Information & communication | 2025 |
| ATH Global Ltd | Wholesale & retail | 2024 |
| Athenax Limited | Information & communication | 2016 |
| Atherstone Place Phase 2 Residents Management Company Limited | Households | 2025 |
| Athletic IQ Group Ltd | Wholesale & retail | 2025 |
| Atholl Rose People Limited | Administrative & support | 2018 |
| ATI Limited | Construction | 2024 |
| Atkins Holdings Hertfordshire Ltd | Real estate | 2023 |
| Atkins Homes and Construction Ltd | Construction | 2023 |
| ATL Tiling Ltd | Construction | 2007 |
| Atlantic House Pension Trustee Limited | Extraterritorial | 1977 |
| Atlas Copco Limited | Administrative & support | 1919 |
| Atlas Copco UK Holdings Limited | Professional & scientific | 1974 |
| Atlas Nova Commerce Ltd | Wholesale & retail | 2025 |
| Atlas&Diyar Ltd | Real estate | 2026 |
| Atoka Services Limited | Construction | 2018 |
| Atria Designs Ltd | Professional & scientific | 2026 |
| Attitude Marketing Ltd | Administrative & support | 2019 |
| Aubade Paris UK Ltd | Administrative & support | 1999 |
| Aubrey Technical Services Ltd | Professional & scientific | 2011 |
| Audio Wireless Limited | Manufacturing | 2009 |
| Augusta Park (Andover) Management Company Limited | Households | 2009 |
| Aura (Isleworth) Residents Management Company Limited | Households | 2007 |
| Aura Autos Limited | Wholesale & retail | 2025 |
| Aurelius International Ltd | Professional & scientific | 2019 |
| Aurizz Constructions Ltd | Construction | 2026 |
| Aurora Art Ltd | Manufacturing | 2024 |
| Austins Residential Lettings Limited | Real estate | 2018 |
| Authentiq Mindset Ltd | Wholesale & retail | 2025 |
| Auto Electrified Ltd | Wholesale & retail | 2022 |
| Auto Provisions Ltd. | Wholesale & retail | 2015 |
| Auto Select Warranties Ltd | Professional & scientific | 2017 |