| ATM Steel Group Ltd | Haverhill | 2018 |
| Atma Consulting Services Limited | Wilmslow | 2024 |
| Atman Eternal Limited | Birmingham | 2019 |
| Atman Group Limited | Wyboston | 2023 |
| Atmd Holdings Limited | London | 2018 |
| Atmere Finance Ltd | Norwich | 2026 |
| Atmo Bidco UK Limited | Cambridge | 2023 |
| Atmos Financial Solutions Ltd | Larkhall | 2023 |
| Atmos Group Ltd | Horsham | 2024 |
| Atmosphere Capital Limited | London | 2017 |
| Atmosphere Solutions Ltd | Sutton Coldfield | 2015 |
| Atmostherm Holdings Limited | Nantwich | 2022 |
| Atmt Holding Limited | London | 2022 |
| ATN Holdings & Commodities Ltd | North Shields | 2023 |
| ATN Investment Holdings, Ltd. | London | 2019 |
| ATN Total Group Limited | Liverpool | 2026 |
| Atnami Ltd | Bromley | 2025 |
| Atodra Investments Ltd | Bolton | 2022 |
| Atog Midco Limited | London | 2019 |
| Atolia Innovations Ltd | Barnsley | 2024 |
| Atoll Capital Limited | Worcester | 2012 |
| Atoll Debtco Ltd | London | 2022 |
| Atoll Midco Ltd | London | 2022 |
| Atom Bank PLC | Newcastle Upon Tyne | 2013 |
| Atom Capital Limited | London | 2006 |
| Atom Finance Ltd | London | 2023 |
| Atom Group Holdings Ltd | Braintree | 2023 |
| Atom Holdco PLC | Newcastle Upon Tyne | 2022 |
| Atom Holdings 2023 Ltd | Ascot | 2023 |
| Atom Holdings Limited | London | 2015 |
| Atom Investors UK Ltd | London | 2025 |
| Atom Juice SHC Holdings Limited | Maidstone | 2024 |
| Atom Partners Limited | Alderley Edge | 2017 |
| Atom Water Holdings Limited | Colchester | 2026 |
| ATOM36 Ltd | London | 2025 |
| Atome PLC | Leeds | 2021 |
| Atome Power Limited | Leeds | 2022 |
| Atomeca Group Ltd | Stockport | 2020 |
| Atomic 42 Ltd | Norwich | 2022 |
| Atomic Bidco 1 Limited | Oxford | 2025 |
| Atomic Engineering Holdings Limited | Solihull | 2012 |
| Atomic EOT Limited | Nottingham | 2023 |
| Atomic Financial Limited | Oxford | 2014 |
| Atomic FX Limited | Liverpool | 2025 |
| Atomic Holdco 1 Limited | Oxford | 2025 |
| Atomic Holdco 2 Limited | Oxford | 2025 |
| Atomic Holdings Ltd | Ashford | 2026 |
| Atomic MC Limited | Nottingham | 2015 |
| Atomic Metals PLC | London | 2025 |
| Atomic Midco 1 Limited | Oxford | 2025 |
| Atomic Midco 2 Limited | Oxford | 2025 |
| Atomic Topco 1 Limited | Oxford | 2025 |
| Atomic Wealth Management Limited | Oxford | 2014 |
| ATOMIC10 Holdings Ltd | Glasgow | 2023 |
| Atomico V Coinvest Limited | London | 2020 |
| Atomico VI Coinvest Limited | London | 2023 |
| Atominvest Limited | London | 2017 |
| Atomix Intelligent Finance Ltd | Bury St. Edmunds | 2022 |
| Atomix Investments Ltd | Bury St. Edmunds | 2026 |
| Atomos Financial Planning Holdings Limited | London | 1999 |
| Atomos Financial Planning Limited | London | 1999 |
| Atomos Investments Holdings Limited | London | 2022 |
| Atomos Investments Limited | London | 1986 |
| Atout Cœur Limited | London | 2026 |
| ATP Accountancy Group Ltd | Manchester | 2023 |
| ATP Adhesive Systems Holding Ltd | Ipswich | 2021 |
| ATP Consultancy Ltd | Burgess Hill | 2024 |
| ATP Media Holdings Limited | Wimbledon | 2017 |
| ATP Media Tennis Limited | Wimbledon | 2017 |
| Atpb Group Limited | London | 2026 |
| Atpc Holding Ltd | Doncaster | 2020 |
| Atph Limited | Chichester | 2022 |
| Atpledge Limited | Manchester | 2012 |
| Atprop Holdings Ltd | Ripley | 2024 |
| Atps Investments and Trading Limited | Poole | 2023 |
| ATR (N.I) Ltd | Armagh | 2022 |
| ATR Holdco Limited | Liverpool | 2017 |
| ATR Holdings Group Ltd | London | 2023 |
| ATR Investments Limited | London | 2015 |
| Atradius Corporate Member Limited | London | 2025 |
| Atradius Pension Trustees Limited | Cardiff | 1991 |
| Atram Financial Ltd | Borehamwood | 2021 |
| Atram Wealth Ltd | London | 2025 |
| Atrato Capital Limited | Watford | 2016 |
| Atrato Onsite Energy Holdco Limited | London | 2021 |
| Atrato Partners Limited | Watford | 2016 |
| Atreides Group Limited | London | 2022 |
| Atri Capital Ltd | London | 2019 |
| Atriafinancial Holdings Ltd | London | 2020 |
| Atrio Holdings Ltd | Brighton | 2025 |
| Atrio Ventures Ltd | Coventry | 2020 |
| Atrium 5 Limited | London | 1993 |
| Atrium Anacott Limited | London | 2023 |
| Atrium Camberley Holdings Ltd | London | 2024 |
| Atrium Holdings Group Ltd | Norwich | 2022 |
| Atrium Insurance Agency Limited | London | 2006 |
| Atrium International Holdings Limited | London | 2023 |
| Atrium Underwriters Limited | London | 1985 |
| Atrium Underwriting Group Limited | London | 1993 |
| Atrium Wealth Management Limited | Bradford | 2017 |