| Atheras Analytics Limited | Information & communication | 2020 |
| ATI Properties Limited | Real estate | 1999 |
| Atilganer Catering and Consulting Limited | Hospitality | 2019 |
| ATK Environmental Ltd | Construction | 2023 |
| Atkor Ltd | Hospitality | 2019 |
| Atlantic and MAY Ltd | Real estate | 2012 |
| Atlantic Sports SL Ltd | Arts & recreation | 2023 |
| Atlas Management Consultants Ltd | Construction | 2023 |
| Atlas Multi Academy Trust | Education | 2011 |
| Atlas Petroleum Ltd | Wholesale & retail | 2025 |
| Atlas Translations Limited | Professional & scientific | 2002 |
| Atlas UK Groups Ltd | Wholesale & retail | 2025 |
| Atlum Limited | Information & communication | 2016 |
| Atmos Electrical Services Limited | Construction | 2022 |
| Atom (London) Limited | Information & communication | 2020 |
| Attention Seeker Ltd | Professional & scientific | 2020 |
| Attewell Hardwicke & Leahy Ltd | Real estate | 2019 |
| Attuned Technology Ltd | Information & communication | 2016 |
| Atwise Ltd | Information & communication | 2023 |
| Aucas Limited | Professional & scientific | 2023 |
| Auckland Operations UK Limited | Other services | 2017 |
| Audio Related Technology Limited | Manufacturing | 2000 |
| Audis Partners Limited | Information & communication | 2018 |
| Audus Holdings Limited | Real estate | 2023 |
| Audus Interiors (Contracts) Ltd | Construction | 2023 |
| Audus Interiors (ST Albans) Ltd | Construction | 2014 |
| Audus Interiors (Windlesham) Ltd | Wholesale & retail | 2023 |
| Audux Ltd | Information & communication | 2023 |
| Aughentaine Consulting LLP | | 2006 |
| August Technologies Ltd | Information & communication | 2022 |
| Aura Luxe Limited | Wholesale & retail | 2020 |
| Aura Property Assets Limited | Real estate | 2017 |
| Auria Clothing Ltd | Wholesale & retail | 2024 |
| Auriel Holidays Limited | Administrative & support | 2020 |
| Auro Apparel Ltd | Wholesale & retail | 2025 |
| Aurora Purple Electrical Limited | Construction | 2021 |
| Aurum Finishing Limited | Manufacturing | 2016 |
| Austen Rental Properties Limited | Real estate | 2015 |
| Authors Aloud UK Limited | Other services | 2015 |
| Auti NOT Naughty Ltd | Hospitality | 2019 |
| Autoclassic Investment Ltd | Extraterritorial | 2014 |
| Autonomate Limited | Information & communication | 2022 |
| Auurah Capital Ltd | Financial services | 2025 |
| AV - Atai 1 LLP | | 2020 |
| AV - Atai 2 LLP | | 2020 |
| AV I CC I LLP | | 2021 |
| AV I COI I LP | | 2021 |
| AV Integration Ltd | Construction | 2013 |
| Avalia Property Ltd | Real estate | 2020 |
| Avalon Property Management Limited | Real estate | 2016 |
| Avanti Enterprises (UK) Limited | Construction | 2008 |
| Avanti MED Ltd | Manufacturing | 2019 |
| Avanza Accounting Limited | Professional & scientific | 2014 |
| AVD Refurbishment Limited | Construction | 2015 |
| Averill Solutions Ltd | Households | 2026 |
| Avesha Optics Ltd | Wholesale & retail | 2023 |
| AVI Charter Ltd | Transport & logistics | 2019 |
| AVI Home Improvement Ltd | Construction | 2015 |
| AVI Home Investments Ltd | Real estate | 2019 |
| Avian Labs Ltd | Information & communication | 2022 |
| Aviva Dentistry Limited | Health & social care | 2008 |
| AVM IT Limited | Information & communication | 2009 |
| AVS Fencing Supplies Limited | Wholesale & retail | 1993 |
| AVW Productions Limited | Wholesale & retail | 2024 |
| Awara Consulting Limited | Professional & scientific | 2017 |
| Awara Holdings Limited | Professional & scientific | 2017 |
| Awas Medics Ltd | Health & social care | 2022 |
| Aweswim Limited | Arts & recreation | 2012 |
| AWI Engineering Solutions Ltd. | Construction | 2022 |
| AWJ Property Investment Ltd | Real estate | 2020 |
| AX Strategy Ltd | Professional & scientific | 2023 |
| Axelrod Properties Limited | Real estate | 2017 |
| Axen Clinics Limited | Health & social care | 2025 |
| Axiom Talent Solutions Ltd | Professional & scientific | 2025 |
| Axis Renovations Ltd | Construction | 2014 |
| Axle Music Limited | Information & communication | 1974 |
| Axton IT Business Consulting Limited | Information & communication | 2011 |
| Axtron Limited | Administrative & support | 2011 |
| Aylard Consulting Limited | Financial services | 2011 |
| Ayot Trading Limited | Construction | 2022 |
| Azamour Online Limited | Information & communication | 2009 |
| Azan and Ahiyan Catering Ltd | Hospitality | 2025 |
| Azfk Properties Ltd | Real estate | 2026 |
| B & a Developers Limited | Construction | 2015 |
| B & B DRY Cleaning Limited | Other services | 2016 |
| B & R Property Living Ltd | Real estate | 2020 |
| B Baker Roofing Limited | Construction | 2015 |
| B E Construction Services Ltd | Construction | 2023 |
| B Hearduk Ltd | Information & communication | 2022 |
| B Lyle Consultancy Limited | Professional & scientific | 2017 |
| B M Pool Heating Limited | Construction | 2023 |
| B Rigging Ltd | Construction | 2023 |
| B Struct Limited | Construction | 2023 |
| B&D Longstone Limited | Real estate | 2021 |
| B&E Consultants Ltd | Construction | 2023 |
| B&F Bridal's Ltd | Wholesale & retail | 2019 |
| B&R Air Conditioning Sales & Service Ltd | Administrative & support | 2017 |
| B&Y Recruitment Ltd | Administrative & support | 2026 |
| B. Maule & Company Limited | Administrative & support | 1981 |
| BA Project Finance Ltd | Administrative & support | 2008 |