| Atomyyx-Removals Limited | Administrative & support | 2016 |
| Atoz Import & Export Ltd | Wholesale & retail | 2023 |
| Atps Lifesciences Ltd | Professional & scientific | 2025 |
| Atralium Limited | Information & communication | 2023 |
| Atrato Capital Limited | Financial services | 2016 |
| Atrato Group Limited | Administrative & support | 2019 |
| Atrato Halliwell Limited | Administrative & support | 2019 |
| Atrato Partners Limited | Financial services | 2016 |
| Atrato Services Limited | Administrative & support | 2018 |
| Atrinet UK Ltd. | Information & communication | 2015 |
| Atrius IT Solutions Ltd | Information & communication | 2021 |
| ATS Electrical and Security Ltd | Construction | 2025 |
| ATS Mini Skips Limited | Water & waste | 2015 |
| Atsaa Consulting Ltd | Information & communication | 2017 |
| Atsaa Trading Ltd | Wholesale & retail | 2020 |
| Attara Properties Ltd | Real estate | 2026 |
| Attic London Ltd | Information & communication | 2011 |
| Atticus Trading International Ltd | Wholesale & retail | 2018 |
| Attlee Bains Churchill Limited | Extraterritorial | 2010 |
| Attune Consulting Ltd | Administrative & support | 2016 |
| Atuendo Ltd | Other services | 2022 |
| ATULKAN0 Ltd | Extraterritorial | 2024 |
| Atvp UK Ltd | Real estate | 2024 |
| Atwell Martin (Holdings) Limited | Real estate | 2008 |
| AU79 Commodities Limited | Financial services | 2008 |
| Auburn Valley Ltd | Information & communication | 2017 |
| Auction Consultants London Limited | Real estate | 2021 |
| Audacious Media Limited | Arts & recreation | 2025 |
| Audio Limited | Manufacturing | 1947 |
| Audio Visual Independent Distribution Limited | Wholesale & retail | 1989 |
| Audiocomp Ltd | Information & communication | 2011 |
| Audit Force Limited | Professional & scientific | 2016 |
| Audit PV Limited | Health & social care | 2012 |
| Augment London Limited | Administrative & support | 2017 |
| Augmentation Consulting Services Limited | Professional & scientific | 2018 |
| Augustus D Consultancy Ltd. | Professional & scientific | 2016 |
| AUM Impax Limited | Professional & scientific | 2026 |
| AUM Sales Ltd | Wholesale & retail | 2026 |
| AUN Fragrances Limited | Wholesale & retail | 2025 |
| Aura & ORE Ltd | Wholesale & retail | 2026 |
| Aura Construction Watford Ltd | Construction | 2025 |
| Aura Education Ltd | Education | 2014 |
| Aura Platform Ltd | Information & communication | 2026 |
| Aura TAN & Beauty Limited | Other services | 2007 |
| Auraflux Consulting Limited | Professional & scientific | 2025 |
| Aurakka Limited | Real estate | 2021 |
| Aurelia Enterprises Limited | | 1974 |
| Aurelio-Veza Ltd | Professional & scientific | 2026 |
| Aureloom Limited | Wholesale & retail | 2025 |
| Aurev Limited | Wholesale & retail | 2025 |
| Aurexxis Limited | Information & communication | 2026 |
| Auri Hair Limited | Other services | 2025 |
| Auric League Limited | Professional & scientific | 2015 |
| Auriol Event Leadership LLP | | 2016 |
| Aurora Medical Services Limited | Health & social care | 2016 |
| Aurora Secrets Limited | Manufacturing | 2018 |
| Aurovic Ltd | Information & communication | 2026 |
| Aurum Collective Ltd | Real estate | 2025 |
| Aurum Enterprise Limited | Wholesale & retail | 2018 |
| Austen Therapies Limited | Health & social care | 2017 |
| Austin Company Of(U.K.)Limited(The) | Construction | 1938 |
| Austins Lane Inc Ltd | Real estate | 2021 |
| Authentic India Mantra Foods Ltd | Hospitality | 2017 |
| Authenticiq Limited | Professional & scientific | 2024 |
| Auto Crash Repair Plus Limited | Wholesale & retail | 2019 |
| Auto Door Systems LLP | | 2020 |
| Auto Electrical Ltd | Wholesale & retail | 2026 |
| Auto Gears Driving School Ltd | Education | 2018 |
| Auto Kenya Ltd | Wholesale & retail | 2007 |
| Auto Safeguard Ltd | Construction | 2024 |
| Auto Salvage Ltd | Wholesale & retail | 2019 |
| Auto Solutions London Ltd | Wholesale & retail | 2018 |
| Autoadi Ltd | Wholesale & retail | 2026 |
| Autoaid Administration Limited | Wholesale & retail | 2026 |
| Autobase Holdings Limited | Wholesale & retail | 2023 |
| Autobase Repair Hub Ltd | Wholesale & retail | 2023 |
| Autodrivexcel UK Ltd | Administrative & support | 2024 |
| Autoflex.Rentals Ltd | Administrative & support | 2024 |
| Autolease and Management Limited | Administrative & support | 2004 |
| Automation Programs Solutions Limited | Information & communication | 2016 |
| Automotive Consultancy Limited | Professional & scientific | 2021 |
| Automotive MLB Group Ltd | Wholesale & retail | 2024 |
| Automover Logistics Ltd | Transport & logistics | 2025 |
| Autonomy Workspace Ltd | Construction | 2011 |
| Autorecrut Limited | Administrative & support | 2021 |
| Autowind Garage Ltd | Wholesale & retail | 2025 |
| Autumn Mist Ltd | Information & communication | 2013 |
| AV Genie Ltd | Information & communication | 2014 |
| AV IT Solutions Ltd | Information & communication | 2018 |
| AVA Developments Limited | Information & communication | 2016 |
| AVA Rose Holdings Ltd | Health & social care | 2021 |
| AVA Stone Group Ltd | Wholesale & retail | 2025 |
| AVA Supplies Ltd | Wholesale & retail | 2024 |
| Avallon Systems Ltd. | Other services | 2007 |
| Avalon Creative Solutions Limited | Information & communication | 2010 |
| Avant-Garde Softwares Ltd | Information & communication | 2023 |
| Avanta Network Ltd | Information & communication | 2023 |
| Avantir Consulting Ltd | Real estate | 2018 |
| Avantis Law Ltd | Professional & scientific | 2022 |
| Avantree Limited | Wholesale & retail | 2014 |