| Atlas Printing Limited | Manufacturing | 2026 |
| Atlas Professionals International Limited | Financial services | 2024 |
| Atlas Professionals UK Limited | Administrative & support | 2006 |
| Atlas Services Group Energy Ltd | Administrative & support | 2004 |
| Atlas Skins & Leather Limited | Wholesale & retail | 2025 |
| Atlas Sporthomes Ltd | Manufacturing | 2018 |
| Atlaseventteam Ltd | Administrative & support | 2023 |
| Atlasud Limited | Transport & logistics | 2018 |
| Atmo Technology Ltd | Professional & scientific | 2015 |
| Atmos Holdings Ltd | Real estate | 2025 |
| Atmos Recruitment Ltd | Administrative & support | 2008 |
| Atny Law Ltd | Professional & scientific | 2025 |
| Atom Acoustics Ltd | Professional & scientific | 2026 |
| Atomic Cleaning and Catering Ltd | Hospitality | 2019 |
| Atomic Cubes Ltd | Wholesale & retail | 2023 |
| Atomic Database Solutions Ltd | Information & communication | 2003 |
| Atomic Mass Limited | Professional & scientific | 2021 |
| Atomic Signs Ltd | Professional & scientific | 2020 |
| Atomic Smash Ltd | Information & communication | 2012 |
| Atour Education Ltd | Education | 2022 |
| ATP Construction South West Ltd | Construction | 2019 |
| ATP Contracts Ltd | Real estate | 2024 |
| ATP Recruitment Ltd | Administrative & support | 2025 |
| ATQ Electrical Ltd | Construction | 2026 |
| ATR Earthworks Limited | Construction | 2024 |
| ATR-7 Limited | Wholesale & retail | 2025 |
| Atria AI Ltd | Information & communication | 2023 |
| Atricure UK Limited | Manufacturing | 2021 |
| ATS Bristol Limited | Hospitality | 2019 |
| ATS Specialists Limited | Professional & scientific | 2012 |
| Attackpests Ltd | Administrative & support | 2010 |
| Attacus Acoustics Ltd | Professional & scientific | 2026 |
| Attenborough Wright Limited | Professional & scientific | 2023 |
| Attendfriend Limited | Professional & scientific | 1935 |
| Attention Industries Limited | Financial services | 2018 |
| Attention Talent Ltd | Administrative & support | 2022 |
| Attentional Ltd. | Information & communication | 1990 |
| Attentoit Ltd | Information & communication | 2017 |
| Attero (Holdings) Limited | Real estate | 2019 |
| Attero Design Limited | Professional & scientific | 2009 |
| Attewell-Smith Limited | Construction | 2011 |
| Attract Electrical Services Ltd | Construction | 2022 |
| Attvance Ltd | Information & communication | 2012 |
| Attwell Developments Ltd | Construction | 2019 |
| Attwell Holdings Ltd | Financial services | 2024 |
| Attwell Homes Ltd | Real estate | 2017 |
| Attwell Projects Ltd | Construction | 2024 |
| Attwoods Property Limited | Professional & scientific | 2009 |
| Atugoasa Limited | Health & social care | 2015 |
| Atunara Group Ltd | Financial services | 2025 |
| Atwell Martin Surveyors (Bristol) Limited | Real estate | 2018 |
| ATX Construction Ltd | Construction | 2025 |
| Atyp Ltd | Information & communication | 2026 |
| ATZ Commercial Ltd | Real estate | 2025 |
| ATZ Homes Limited | Construction | 2009 |
| Aube Holding Ltd | Financial services | 2022 |
| Aubert North Limited | Professional & scientific | 2013 |
| Auburn Space Consulting Limited | Professional & scientific | 2015 |
| Auburn Windows Ltd | Construction | 2023 |
| AUC Sales Limited | Wholesale & retail | 2014 |
| Auction House at Harbour Taverns Ltd | Hospitality | 2025 |
| Auction Sales Ltd | Professional & scientific | 2012 |
| Auctioneum Ltd | Wholesale & retail | 2012 |
| AUD Projects Ltd | Professional & scientific | 2023 |
| Audax Management Company (UK) Limited | Financial services | 2022 |
| Audazzle Ltd | Information & communication | 2014 |
| Audeo Capital Limited | Financial services | 2019 |
| Audi-Volks Services (A&B) Ltd | Wholesale & retail | 2023 |
| Audi-Volks Services (SW) Ltd | Wholesale & retail | 2011 |
| Audiac Audio Ltd | Manufacturing | 2018 |
| Audio & Lighting Innovations Limited | Professional & scientific | 2026 |
| Audio Excellence Limited | Wholesale & retail | 1974 |
| Audio Logical Group Limited | Arts & recreation | 2015 |
| Audio Swann Ltd. | Information & communication | 2014 |
| Audio T (Oxford) Limited | Wholesale & retail | 2012 |
| Audio T (UK) Limited | Wholesale & retail | 1997 |
| Audio T Custom Limited | Wholesale & retail | 2006 |
| Audio T Holdings Limited | Professional & scientific | 2008 |
| Audio T Limited | Wholesale & retail | 1990 |
| Audio Uproar Limited | Information & communication | 2009 |
| Audiotarky Ltd | Information & communication | 2020 |
| Audley Advisory Limited | Professional & scientific | 2014 |
| Audley Lodge Residents Limited | Households | 1989 |
| Auev Consulting Ltd | Administrative & support | 2022 |
| Augment Computing UK Ltd | Administrative & support | 2025 |
| Augmental Limited | Information & communication | 2023 |
| August Entertainment Limited | Information & communication | 2026 |
| August Ventures Ltd | Health & social care | 2025 |
| Augustin Dick Ltd | Health & social care | 2021 |
| Augustines Injury Law Limited | Professional & scientific | 2004 |
| Augustus Properties Limited | Real estate | 2019 |
| Augustus Sets Limited | Construction | 2014 |
| Aukacer Limited | Construction | 2025 |
| AUM South West Ltd | Wholesale & retail | 2021 |
| AUM Southwest News Ltd | Wholesale & retail | 2025 |
| Aumkar Holdings Limited | Construction | 2024 |
| Aura & Thread Ltd | Professional & scientific | 2026 |
| Aura Analytics Ltd | Information & communication | 2024 |
| Aura Data Ltd | Information & communication | 2019 |
| Aura Ethical Trading Ltd | Wholesale & retail | 2018 |