| Associated Farming Groups Ltd | Newry | 2002 |
| Association for Animals Horticultural & Equine Practitioners CIC | Tonbridge | 2019 |
| Association of Protecting Exotic Species CIC | St. Helens | 2025 |
| Assured Agronomy Ltd | Scunthorpe | 2014 |
| Assured Technical Services Ltd | Glasgow | 2016 |
| Assynt Crofters Trust Limited | Lairg | 1992 |
| Astarte Aquaria Ltd | Upper Bathville | 2025 |
| Astaxanthin Ltd | Alcester | 2025 |
| Astbury Sport Horses Ltd | Prestwich | 2021 |
| Astbury Tree Care Limited | Northampton | 2024 |
| Aster Horses Ltd | Ashford | 2016 |
| Asthall Wild CIC | Burford | 2023 |
| Asthally Limited | Witney | 1992 |
| Astley Church Farm Ltd | Nuneaton | 2020 |
| Astley Livestock Ltd. | Morpeth | 2012 |
| Astley Moss Equine Bedding Ltd | Leigh | 2026 |
| Astley Rathbone Farming Ltd | Newark | 2020 |
| Aston Eyre Farms Salop Ltd | London | 2025 |
| Aston Park Angling Centre Ltd | Aston | 2018 |
| Aston Park Fisheries Limited | Sheffield | 2002 |
| Aston Vineyard Ltd. | Harrow | 2023 |
| Astrid Shipping Company Limited | London | 2012 |
| Astronaut Shellfish Limited | Lochs | 2012 |
| Astrop Farms Limited | Banbury | 1995 |
| Astrum EQ Limited | Wirral | 2025 |
| Astwell Castle Farm Limited | Brackley | 1956 |
| Astwood Cattery Limited | Redditch | 2017 |
| Astwood Livery Limited | Redditch | 2008 |
| Asureq Agricultural Limited | Prescot | 2023 |
| Aswarby Estate Farms Limited | London | 2012 |
| Aswardby Park Farms Limited | Spilsby | 1982 |
| AT Capital Management Ltd | Hadleigh | 2020 |
| AT Contracting Ltd | Pulborough | 2024 |
| AT Contracting South Lakes Ltd | Kendal | 2026 |
| AT Eco Ltd | Windsor | 2022 |
| AT Fisheries Ltd | Livingston | 2024 |
| AT Threlkeld Limited | Ulverston | 2012 |
| Atacama Textiles Limited | North Chailey | 2002 |
| Atala Limited | Ripon | 2007 |
| Atavita Foods Ltd | Cardiff | 2026 |
| ATC Equestrian Ltd | Cambridge | 2023 |
| ATC Southern Limited | Oxted | 2003 |
| ATD Agri Ltd | Bishop Auckland | 2023 |
| ATD Agricultural Ltd | Boston | 2023 |
| ATD Equine Services Ltd | Kinross | 2024 |
| Atdunn Limited | Taunton | 2025 |
| Atelier 52 Limited | London | 2022 |
| Atelier Equine Ltd | Daventry | 2024 |
| Atfb Limited | Redruth | 2008 |
| ATG Agricultural Contractors Limited | Crewkerne | 2024 |
| Athaday Limited | Cirencester | 2001 |
| Athelas Plants Limited | Battle | 2021 |
| Atheralls Farms Limited | Hove | 1984 |
| Atherden Racing Farriers Ltd | Towcester | 2024 |
| Atherton Group Holdings Limited | Oswestry | 2018 |
| Athertrend Limited | Northampton | 1998 |
| Atikan Ltd | Dalry | 2022 |
| ATK Arboriculture Ltd | Rugby | 2018 |
| Atkin & Farrow Limited | Skegness | 1958 |
| Atkin Property Ltd | Dumfries | 2022 |
| Atkins & Candler ARB Ltd | Bishops Stortford | 2023 |
| Atkinson Earthmoving Contracts Ltd | Kirkby Stephen | 2019 |
| Atkinson Tree Services Ltd | Teignmouth | 2016 |
| Atkinsons (Aberford) Limited, | Near Leeds. | 1963 |
| Atkinsons Farms Limited | Wainfleet St Mary Lincolnshire | 1955 |
| ATL UK Enterprises Limited | Manchester | 2019 |
| Atlanta 11 Limited | Bridlington | 1999 |
| Atlantic Aquatics Ltd | London | 2023 |
| Atlantic Blue Marlin Ltd | Monmouth | 2015 |
| Atlantic Dressage Ltd | Wellesbourne | 2026 |
| Atlantic Fungi Coop Limited | Bude | 2023 |
| Atlantic Gate Ltd | Truro | 2006 |
| Atlantic Mariculture Ltd | Edinburgh | 2018 |
| Atlantic Mussels Limited | Walls | 2011 |
| Atlantic Sea Gold Ltd | Ardglass | 2022 |
| Atlantic Thunnus Limited | London | 2026 |
| Atlantic Trawlers Kilkeel Limited | Lisburn | 2011 |
| Atlantis Reef Emporium Ltd | London | 2026 |
| Atlas Arboriculture Ltd | Chelmsford | 2019 |
| Atlas Automated Services Ltd | Wantage | 2024 |
| Atlas Resources Management Limited | London | 2026 |
| Atlas Tractors Ltd | Oswestry | 2009 |
| Atlas Tree Care Limited | Salisbury | 2008 |
| Atlas Tree Surgery Limited | Cambridge | 2007 |
| Atlex Ltd | Surbiton | 2024 |
| ATN Farms Limited | Buckingham | 2016 |
| ATO Holdings Limited | Luton | 1951 |
| Atom Tree Surgery Ltd | St. Leonards-On-Sea | 2008 |
| Atrahasis Limited | Gloucester | 2023 |
| Atran Forestry Services Ltd | Musselburgh | 2023 |
| Attain Fishing Limited | Hessle | 2013 |
| Attillo Limited | Droitwich | 1987 |
| Attrell Tree Surgery Limited | Waterlooville | 2014 |
| Attwell & SON Ltd | Warwick | 2022 |
| Attwell Farm Limited | Redditch | 1998 |
| Attwell Monks Ltd | Uckfield | 2021 |
| ATV Climate Solutions Ltd | London | 2016 |
| Atvantage Limited | Dunfermline | 2012 |
| Auberies Estate Farms Limited | Sudbury | 1964 |
| Aubin Rural Ltd | Hereford | 2023 |