| Assist AID Ltd | Information & communication | 2024 |
| Assist India | Water & waste | 2021 |
| Assurance Innovation Ltd | Professional & scientific | 2016 |
| Assure Care Ltd | Health & social care | 2013 |
| Assure Consulting (Bedford) Limited | Real estate | 2011 |
| Assure Consulting (Holdings) Limited | Professional & scientific | 2014 |
| Assure Consulting EOT Limited | Other services | 2023 |
| Assure Property Management Ltd | Real estate | 2026 |
| Assured Business Support Limited | Construction | 2014 |
| Assured Care Living Ltd | Health & social care | 2023 |
| Assured Care Services (MK) Ltd | Public administration | 2021 |
| Assured Holdings Limited | Real estate | 1952 |
| Assured Workplace Services Limited | Professional & scientific | 2013 |
| Assurium Limited | Information & communication | 2014 |
| AST Garage Ltd | Wholesale & retail | 2016 |
| Asta Intl Ltd | Wholesale & retail | 2011 |
| Astaan Limited | Information & communication | 2025 |
| Astellas Accounting Solutions Ltd | Professional & scientific | 2025 |
| Astellon Limited | Administrative & support | 2011 |
| Aster Creative Limited | Professional & scientific | 2023 |
| Aster Development Company Limited | Real estate | 1978 |
| Asteria MK Limited | Hospitality | 2024 |
| Astle Limited | Wholesale & retail | 1973 |
| Aston Accountancy Limited | Professional & scientific | 1993 |
| Aston Black Limited | Professional & scientific | 2005 |
| Aston Data & Electrical Ltd | Information & communication | 2015 |
| Aston Engineering Consultants Limited | Construction | 2018 |
| Aston Housing Limited | Real estate | 2026 |
| Astra Global Ventures Ltd | Wholesale & retail | 2025 |
| Astra Technologies Group Ltd | Information & communication | 2026 |
| Astral Consulting Limited | Information & communication | 1999 |
| Astral Mandala Foundation Limited | Health & social care | 2025 |
| Astral Tech Limited | Information & communication | 2005 |
| Astraline Logistics Limited | Wholesale & retail | 2017 |
| Astralmandala Limited | Administrative & support | 2025 |
| Astralweb Technology Limited | Information & communication | 2013 |
| Astrateq Solutions Ltd | Information & communication | 2021 |
| ASV International Ltd | Professional & scientific | 2021 |
| Asvini Ltd | Transport & logistics | 2025 |
| ASW Engineering Ltd | Professional & scientific | 2023 |
| AT Elite Limited | Real estate | 2023 |
| AT Gill Empire Ltd | Real estate | 2023 |
| AT Gill Enterprises Investments Ltd | Financial services | 2024 |
| AT Home Coatings Ltd | Construction | 2023 |
| AT Your Door Step Services Ltd | Administrative & support | 2025 |
| Atat Ansah Group Limited | Financial services | 2024 |
| Ataz Properties Ltd | Real estate | 2024 |
| ATB Marketing Ltd. | Professional & scientific | 2022 |
| ATB Trading & Marketing Ltd | Wholesale & retail | 2022 |
| ATC I.T Ltd | Information & communication | 2018 |
| ATC Music Merch Ltd | Wholesale & retail | 2023 |
| ATD Global Solutions Limited | Professional & scientific | 2025 |
| Atelier Automotive Art Ltd | Wholesale & retail | 2018 |
| Atelier Automotive London Limited | Wholesale & retail | 2025 |
| Atelier Creative Partners Ltd | Information & communication | 2023 |
| Atelis Aspecto Ltd | Wholesale & retail | 2022 |
| Aten Logistics Limited | Administrative & support | 2015 |
| Atera Cybernet Ltd | Information & communication | 2024 |
| ATF Finance Ltd | Financial services | 2014 |
| ATH Invest Direct Ltd | Real estate | 2022 |
| Athiri Group Limited | Manufacturing | 2012 |
| Athiyen Ltd | Hospitality | 2025 |
| Athletic Development LAB Ltd | Education | 2026 |
| Athletic Performance Academy (APA) Limited | Arts & recreation | 2011 |
| Athletiq Ltd | Information & communication | 2026 |
| Athlon Mobility Services UK Limited | Financial services | 1998 |
| Athos Properties Ltd | Real estate | 2022 |
| Athri & Aurva Limited | Information & communication | 2015 |
| ATI Auto Ltd | Wholesale & retail | 2016 |
| Atika Homes Limited | Real estate | 2025 |
| Atika Solutions Ltd | Information & communication | 2017 |
| Atila Consulting Ltd | Professional & scientific | 2022 |
| Atila Group Limited | Real estate | 2025 |
| ATK Limited | Information & communication | 2009 |
| Atkin-Smith & Associates Ltd | Administrative & support | 2020 |
| Atkinson Family Wines Limited | Manufacturing | 2022 |
| Atkinson House (Worthing) Limited | Real estate | 2011 |
| Atko Consulting Ltd | Professional & scientific | 2014 |
| Atlantic Data Ltd | Information & communication | 2000 |
| Atlantis Software Limited | Professional & scientific | 2019 |
| Atlas Design & Build Limited | Construction | 2016 |
| Atlas Family Partners Limited | Financial services | 2015 |
| Atlas Forge Ltd | Information & communication | 2025 |
| Atlas Triathlon Ltd | Wholesale & retail | 2025 |
| Atlas Windows & Maintenance Ltd | Construction | 2021 |
| Atlas Workplace Services Limited | Administrative & support | 2012 |
| ATM29 Ltd | Information & communication | 2021 |
| Atma Happy Limited | Other services | 2024 |
| Atodiresei Ionut Transport Ltd | Transport & logistics | 2016 |
| Atomic Flooring LLP | | 2009 |
| Atomic Studios & Partners Limited | Arts & recreation | 2021 |
| Atoz Design Limited | Wholesale & retail | 2014 |
| Atoz Impex Limited | Other services | 2022 |
| ATP Aircraft Engineering Ltd | Manufacturing | 2014 |
| Atpoult Ltd | Information & communication | 2020 |
| Atrus Ltd | Professional & scientific | 2021 |
| Atta Ventures Ltd | Information & communication | 2025 |
| Attenborough Productions Limited | Information & communication | 2016 |
| Attendite Ltd | Information & communication | 2019 |
| Attendite Realty Ltd | Real estate | 2022 |