| Atrium Gantrys Maintenance Limited | Bickley | 1991 |
| Atrivita Limited | London | 2013 |
| Atrone Tech Ltd | Letchworth Garden City | 2025 |
| Atropakan Limited | Oxford | 2019 |
| Atrshield Ltd | Leeds | 2021 |
| Atrumedia Ltd | Ruislip | 2024 |
| Atrytone Creations Ltd | Camberley | 2026 |
| ATS (Folkestone) Ltd | Folkestone | 1999 |
| ATS Applied Tech Systems Scotland Ltd | Broxburn | 2003 |
| ATS Auto Technology Limited | London | 2023 |
| ATS Aviation Limited | Wallsend | 2015 |
| ATS Cycles Limited | Macclesfield | 2014 |
| ATS Digital Solutions Limited | Chippenham | 2023 |
| ATS Engineering Ltd | Rotherham | 2017 |
| ATS Headwear & Accessories Ltd | Pontypridd | 2026 |
| ATS Heavy Equipment Engineers Ltd | Wakefield | 2024 |
| ATS Machinery Limited | Horsham | 2003 |
| ATS Mouldings Ltd | Maidstone | 1979 |
| ATS Plastics Limited | Bromsgrove | 2015 |
| ATS Trusses Ltd | Craigavon | 2011 |
| Atss Group Limited | London | 2022 |
| Atstiches Ltd | Glasgow | 2023 |
| Atstone Memorials Limited | Sudbury | 2019 |
| Atsuko Kudo Limited | London | 2010 |
| ATT Advanced Control Technology Ltd | Bury St. Edmunds | 2026 |
| ATT Engineering Ltd | Neath | 2010 |
| ATT Fabrications Limited | Spennymoor | 2008 |
| ATT Systems Limited | Derby | 2020 |
| ATT Walters Limited | Crawley | 2015 |
| ATT Works Ltd | London | 2026 |
| Atta Forever Limited | Harrogate | 2020 |
| Attach-It Limited | Middlewich | 2015 |
| Attache Accessories Ltd | Twickenham | 2026 |
| Attachment Solutions Ltd | Colchester | 2010 |
| Attacksense Ltd | Astwood | 2019 |
| Attain WSS Global Holdings Limited | Leeds | 2018 |
| Attenborough Brush Limited | Nottingham | 2000 |
| Attenborough Dental Laboratories Ltd | Nottingham | 2000 |
| Attenborough Direct Ltd | Nottingham | 2001 |
| Attenborough Doors Limited | Ilkeston | 1986 |
| Attenborough Limited | Nottingham | 1935 |
| Attenborough Medical Limited | Nottingham | 2021 |
| Attends Healthcare Finance Limited | Wakefield | 2007 |
| Attends Healthcare Limited | Wakefield | 2007 |
| Attention 2 Detail - Creative Marketing & Print Solutions Limited | Bath | 2001 |
| Attention Span Limited | London | 2022 |
| Attention to Detail (Fabrications) Limited | Newark | 2020 |
| Attentional Machines Limited | Rotherham | 2010 |
| Attercliffe Consultants Limited | Sheffield | 2017 |
| Atterton and Ellis Limited | Haverhill | 1941 |
| Attewell Limited | Hayes | 1962 |
| Attey Engineering Limited | Llanrwst | 2016 |
| Attibear Limited | Malmesbury | 2020 |
| Attic Barrel Store JQ Ltd | Oldbury | 2022 |
| Attic Brew Co. Ltd | Oldbury | 2018 |
| Attica Components Limited | Swindon | 1994 |
| Attica Studios Limited | Horsham | 2022 |
| Atticus Line Limited | Maidenhead | 2024 |
| Attilios Coffee House Limited | London | 2015 |
| Attire Emea Ltd | Birmingham | 2012 |
| Attis Tents Ltd | London | 2021 |
| Attitude Choppers Ltd | Southampton | 2024 |
| Attitude Custom Cycles Ltd | Southampton | 2016 |
| Attitude Customs Ltd | London | 2019 |
| Attitude Cycles Ltd | Bradford | 2016 |
| Attitude Hair and Beauty Ltd | London | 2021 |
| Attitude Media Group Ltd | Holywood | 2025 |
| Attleborough Foods Limited | Uxbridge | 2021 |
| Attlebridge Concrete Products Limited | Lowestoft | 2017 |
| Attmu Co., Ltd | West Midlands, England | 2025 |
| Attock Beds Limited | Bradford | 2025 |
| Attollo Artisan Ltd | Redruth | 2026 |
| Attollo Lifts Ltd | Swanscombe | 2022 |
| Attorcoppe Ltd | London | 2025 |
| Attractions Digital Print & Sign Ltd | Houghton Le Spring | 2024 |
| Atts'Amore Inspired Products Ltd | London | 2024 |
| Attunor Limited | London | 2011 |
| Attur Sound and Production Ltd | Nottingham | 2023 |
| Attwater & Sons Limited | Preston | 1939 |
| Attwell Autos Limited | Leigh On Sea | 2000 |
| Attwood Product Development Ltd | Wolverley | 2023 |
| Atuk Ltd | Great Yarmouth | 2016 |
| Atul Agro Techno Limited | Greater Manchester | 2009 |
| Atum Stone Ltd | Bath | 2020 |
| Atumon Limited | Belfast | 2024 |
| ATV North Limited | Bishop Auckland | 2003 |
| Atvb Ltd | Aylesbury | 2015 |
| Atvocal Ltd | Hornchurch | 2022 |
| ATW Carpentry & Building Ltd | Epsom | 2026 |
| ATW Carpentry and Design Ltd | Romford | 2024 |
| Atwell International Limited | Worcester | 1996 |
| Atwell Limited | Hitchin | 1989 |
| Atwell Properties Ltd | Scunthorpe | 2023 |
| Atwud Engineering Services Ltd | Sheffield | 2010 |
| Atx-Dyno Ltd. | Oldham | 2015 |
| Atyab Tamim Limited | Slough | 2024 |
| Atypic Skincare Ltd | Harrow | 2016 |
| Atyre Consulting Limited | London | 2012 |
| Atz-Motion Ltd | Colchester | 2024 |
| AU Aviation Ltd | Cambridge | 2026 |