| Applied Performance Ltd | Professional & scientific | 2023 |
| Apply NOW Ltd | Professional & scientific | 2023 |
| Appssential Limited | Information & communication | 2012 |
| Aqbio Limited | Professional & scientific | 2018 |
| Aqgsa Limited | Administrative & support | 2013 |
| Aquaffirm Limited | Professional & scientific | 2014 |
| Aquila Advisory Limited | Professional & scientific | 2010 |
| Aquila Art Advisory Ltd | Wholesale & retail | 2025 |
| Aquila Build Limited | Construction | 2024 |
| Aquitude Group Ltd | Information & communication | 2019 |
| Aquiva Foundation | Water & waste | 2011 |
| AR General Ltd | Construction | 2026 |
| Arbileo Advisory Ltd | Professional & scientific | 2020 |
| Arbitrarj Limited | Other services | 2015 |
| Arbitration Advisory Ltd | Administrative & support | 2020 |
| Arbor Environmental Ltd | Agriculture & fishing | 2021 |
| ARC Installations Ltd | Construction | 2020 |
| Arcadia Court Residents Association Limited | Households | 2000 |
| Arcanaset Property Management Limited | Real estate | 1994 |
| Arcbridge Consulting Ltd | Professional & scientific | 2025 |
| Archer Audio Limited | Arts & recreation | 2022 |
| Archer Travel (UK) Limited | Administrative & support | 2023 |
| Arctic Flow AI Limited | Information & communication | 2025 |
| Arda Future Ltd | Information & communication | 2026 |
| Arda Trend Ltd | Wholesale & retail | 2024 |
| Ardau Community Waste Power Limited | Professional & scientific | 2018 |
| Arden London Ltd | Professional & scientific | 2026 |
| Ardhi Digital Consulting Limited | Information & communication | 2021 |
| Area H20 Limited | Wholesale & retail | 2008 |
| Aren & Adel Limited | Wholesale & retail | 2024 |
| Arete Centric Limited | Professional & scientific | 2014 |
| Aretework LLP | | 2011 |
| Argenta IS Back! Ltd | Administrative & support | 2023 |
| Argentic Limited | Other services | 2023 |
| Arghgee EC Ltd | Professional & scientific | 2025 |
| Arhz Homes Ltd | Real estate | 2021 |
| Aria Shipping Limited | Transport & logistics | 2025 |
| Ariston Village Pizza Ltd | Hospitality | 2023 |
| ARK Systems Limited | Professional & scientific | 2014 |
| Arka Projects Ltd | Professional & scientific | 2017 |
| Arkia Limited | Professional & scientific | 2017 |
| Arkle Moor Riding Centre Ltd | Agriculture & fishing | 2022 |
| Arklebeck Ltd | Information & communication | 2017 |
| Arlena Construction and Electrical Consultancy Ltd | Construction | 2020 |
| Arlington Ford Limited | Professional & scientific | 2011 |
| Armada Development Limited | Real estate | 2023 |
| Arna Accounting Services Ltd | Professional & scientific | 2023 |
| Aronet-International Ltd | Information & communication | 2025 |
| Arragon Road Residents Limited | Households | 1996 |
| Art Below Limited | Arts & recreation | 2006 |
| Art House London Limited | Other services | 2014 |
| Art Hub Gallery Ltd | Wholesale & retail | 2024 |
| Art Vandelay Oracles Limited | Information & communication | 2008 |
| Art Wars Limited | Arts & recreation | 2021 |
| ART2APP Ltd | Information & communication | 2023 |
| ART8 Saloon Limited | Other services | 2015 |
| Artbeep Ltd | Wholesale & retail | 2017 |
| Articlus Limited | Professional & scientific | 2017 |
| Articulate Multilingual Ltd | Professional & scientific | 2022 |
| Artley Holdings Limited | Real estate | 2025 |
| Artliner Ltd | Other services | 2025 |
| Arun Electric Ltd | Construction | 2014 |
| Asbru Ltd | Wholesale & retail | 1999 |
| ASCOT431 Ltd | Wholesale & retail | 2026 |
| Asgard.World Holdings Ltd | Financial services | 2023 |
| Asgard.World Ltd | Information & communication | 2022 |
| Ashbury London Ltd | Households | 2021 |
| Ashchase Trade Ltd | Wholesale & retail | 2016 |
| Ashcourt Builders Ltd | Construction | 2011 |
| Ashcourt LER Ltd | Construction | 2024 |
| Ashdown Treesurgeons Limited | Agriculture & fishing | 2019 |
| Ashley Fuller Tennis Solutions Limited | Arts & recreation | 2010 |
| Asim Property Investments Limited | Real estate | 2019 |
| Asiri Perera Photography Ltd. | Professional & scientific | 2018 |
| Asnm Property Maintenance Ltd | Administrative & support | 2025 |
| Aspecta Group Limited | Wholesale & retail | 2026 |
| Asquire Ventures Limited | Administrative & support | 2016 |
| Asquire Wealth Management Limited | Financial services | 2013 |
| Asset Location Ltd | Real estate | 2013 |
| Association of Azerbaijani British Professionals Aabp Community Interest Company | Professional & scientific | 2024 |
| Associo Ltd | Information & communication | 2018 |
| Astn ED Ltd | Education | 2026 |
| Astra Property Ltd | Administrative & support | 2017 |
| Astrameta Limited | Wholesale & retail | 2021 |
| Astrato Analytics Ltd. | Information & communication | 2023 |
| Aswespeak Limited | Professional & scientific | 2002 |
| AT Electrical Services Limited | Construction | 2009 |
| AT Speed IT Ltd | Information & communication | 2014 |
| ATA Circle Ltd | Transport & logistics | 2025 |
| ATA Global Horizons Ltd | Professional & scientific | 2019 |
| Ateavesta Ltd | Wholesale & retail | 2019 |
| Atelier BE Ltd | Professional & scientific | 2025 |
| ATF Land Ltd | Construction | 2019 |
| ATH an Chuillean Services Ltd | Other services | 2021 |
| Athar Group Ltd | Professional & scientific | 2026 |
| Athea Property Investment Ltd | Households | 2019 |
| Atheon Ltd | Wholesale & retail | 2026 |
| ATK Consultants Limited | Administrative & support | 2020 |
| Atlantis Health UK Limited | Health & social care | 2006 |
| Atlantis Legal Ltd | Professional & scientific | 2025 |