| Agilitrix UK Ltd | Information & communication | 2022 |
| AGL Group Holdings Ltd | Construction | 2024 |
| AGL Group Ltd | Construction | 2024 |
| AGT Electrical Services Ltd | Construction | 2022 |
| AH Holdings Group Ltd | Financial services | 2025 |
| AHS Monitoring Limited | Wholesale & retail | 2012 |
| Air CNC Ltd | Administrative & support | 2019 |
| Aircon Elite Limited | Construction | 2015 |
| AJ Soft Ltd | Information & communication | 2017 |
| AJB Property & Landscape Improvements Ltd | Construction | 2025 |
| AJD Solutions Ltd | Information & communication | 2017 |
| AJT Promotions Limited | Arts & recreation | 2022 |
| AK Business Empire Company Ltd | Professional & scientific | 2024 |
| AKA Structures Limited | Construction | 2023 |
| Akshar Productions Ltd | Administrative & support | 2025 |
| AKY Medical Limited | Health & social care | 2013 |
| Alami International Limited | Wholesale & retail | 1973 |
| Alan Farlie Design Ltd | Professional & scientific | 2025 |
| Alaz UK Properties Ltd | Real estate | 2024 |
| ALB Security & Construction Limited | Administrative & support | 2022 |
| Alban Cleaning Services Limited | Administrative & support | 1997 |
| Albie Auto's Limited | Wholesale & retail | 2008 |
| Albion Stride Limited | Information & communication | 2024 |
| Alders Court Estate Limited | Real estate | 1996 |
| Alelectrical (Herts) Limited | Construction | 2024 |
| Aletrari Property Ltd | Real estate | 2025 |
| Alex GOY Limited | Professional & scientific | 2018 |
| Alexander Financial Ltd | Financial services | 2007 |
| Alexander Oliver Design Limited | Administrative & support | 2024 |
| Alfresco Collection Ltd | Wholesale & retail | 2025 |
| ALG Neckwear Ltd | Wholesale & retail | 2015 |
| ALH Consulting Services Ltd | Professional & scientific | 2024 |
| Alicegate Limited | Health & social care | 2013 |
| Aliviar Financial Ltd | Administrative & support | 2020 |
| Alkazar Limited | Information & communication | 2024 |
| ALL 4 GAS Ltd | Construction | 2017 |
| ALL About ME Salon Ltd | Other services | 2013 |
| ALL Airports Taxi Ltd | Transport & logistics | 2025 |
| Alletun Limited | Financial services | 2025 |
| Aloco Group Limited | Financial services | 2018 |
| Aloco Limited | Professional & scientific | 2002 |
| Alpha Auto World Ltd | Wholesale & retail | 2023 |
| Alterra Business Consulting Ltd | Administrative & support | 2005 |
| AMA Louise Ltd | Arts & recreation | 2024 |
| Amalis Construction Limited | Construction | 2017 |
| Ambar Kelly Aviation Limited | Transport & logistics | 2016 |
| Ambar Kelly Limited | Manufacturing | 2003 |
| American Motorhome Club UK Limited | Other services | 2023 |
| Amiable Services Ltd | Real estate | 2018 |
| Amoyensis Limited | Financial services | 2024 |
| Andallus Limited | Information & communication | 2000 |
| Andy Davies Furniture Ltd | Construction | 2023 |
| Angel of the North Ltd | Information & communication | 2019 |
| Anglian Access Repairs Ltd | Manufacturing | 2019 |
| Anglo American Optical Limited | Manufacturing | 1999 |
| Anglo Latin American Foundation | Health & social care | 1998 |
| Anime Clothing Limited | Wholesale & retail | 2024 |
| Anke Ltd | Construction | 2016 |
| ANN Designer Living Limited | Professional & scientific | 2011 |
| Annabella Moore Ltd | Wholesale & retail | 2017 |
| Anyfoam Ltd | Wholesale & retail | 2012 |
| ANZ Digital Limited | Information & communication | 2014 |
| Apexgrange Limited | Wholesale & retail | 1993 |
| Apollo QS Consultancy Ltd | Professional & scientific | 2023 |
| Appey Studios Ltd | Information & communication | 2013 |
| Appleby Grove (Welwyn Garden City) Management Company Limited | Households | 2014 |
| Applecroft Cottages Residents Association Limited | Households | 1984 |
| Applecroft School | Education | 2012 |
| Apprenticeship Central 2024 Ltd | Administrative & support | 2024 |
| Apterve Limited | Information & communication | 2012 |
| Aquablock Limited | Manufacturing | 2023 |
| Aquila Editing Associates Ltd | Information & communication | 2025 |
| Aqweela Education Ltd | Education | 2022 |
| AR EOT Holdings Ltd | Financial services | 2024 |
| AR GP Group Ltd | Wholesale & retail | 2024 |
| Aray Electrical Ltd | Construction | 2018 |
| Arch Tech Ltd | Administrative & support | 2025 |
| Ardale (Oakham) Limited | Health & social care | 2019 |
| Ardale (Potters Bar) Limited | Health & social care | 2023 |
| Ardale (Potters Grange) Limited | Health & social care | 2020 |
| Ardale (Welwyn Grange) Limited | Health & social care | 2020 |
| ARG Electrics Ltd | Construction | 2026 |
| Aries Management Group Ltd | Other services | 2017 |
| Ariso Ltd | Professional & scientific | 2020 |
| Aristo Trading Import and Export Ltd | Wholesale & retail | 2026 |
| Aristo Tradinh Ltd | Wholesale & retail | 2026 |
| ARK Autos Limited | Wholesale & retail | 2023 |
| Arka Shine Ltd | Professional & scientific | 2017 |
| Armitage Finance Unlimited | Administrative & support | 2006 |
| Arrimo Ltd | Information & communication | 2023 |
| Art by Diah Ltd | Arts & recreation | 2021 |
| Art Commercial Interiors Ltd | Construction | 2022 |
| Arts AMK Group Ltd | Wholesale & retail | 2025 |
| Arts Constructions Ltd | Construction | 2013 |
| Artur Lifts Services With Moustache Ltd | Manufacturing | 2025 |
| AS Herts Consulting Ltd | Information & communication | 2021 |
| Asasika Group Limited | Households | 2022 |
| ASH Tree Productions Ltd | Professional & scientific | 2016 |
| Ashbys National Stocktakers Limited | Other services | 2017 |
| Asher Amarachi Ltd | Education | 2024 |