| Ashes Farm Limited | Ruswarp | 2014 |
| Ashlar Stone Products Limited | Keighley | 2019 |
| Ashrise Properties Limited | Lincolnshire | 2003 |
| Ashton Bulk Limited | Somerset | 2007 |
| Ashtown Intervention & Completion Services Ltd | Aberdeen | 2025 |
| Ashworth Waste Services Ltd | Scarborough | 2019 |
| Asia Pacific Energy Corporation Ltd | London | 2017 |
| Asia Power Ltd | London | 2014 |
| Asmart ATJ Recovery Limited | Bracknell | 2003 |
| Asmeh Europe Industries Limited | Cambridge | 2025 |
| ASP Rope Access UK Limited | New Town | 2021 |
| Asphalt Burner Services Limited | Birmingham | 2005 |
| Aspire Control Ltd | Orpington | 2016 |
| Assala Energy UK Limited | London | 2017 |
| Asset Management Fidelity Ltd | Manchester | 2018 |
| Associated Polymer Services Group Ltd | Sheffield | 2024 |
| Associated Polymer Services Limited | Sheffield | 1995 |
| Assos Tech Limited | Edinburgh | 2014 |
| Assynt Geoconsulting Limited | Glasgow | 2015 |
| Asteroids Mining Ltd | Ilford | 2025 |
| Astley Sand & Aggregates Limited | Manchester | 1973 |
| Aston Inspection Services Ltd | Houghton Le Spring | 2017 |
| Astor Energetics Ltd | Southampton | 2025 |
| Astrid Europe Ltd | Cardiff | 2026 |
| Astrium Energy UK Ltd | Widnes | 2024 |
| Asuzu (UK) Limited | Ilford | 2008 |
| AT Marine Ltd | Cardiff | 2016 |
| Atacama Gold Limited | London | 2018 |
| Atana Elements UK Ltd | London | 2022 |
| Atana Elements UK PC S-1 Ltd | London | 2023 |
| ATB Consortium Group Limited | London | 2011 |
| Atcom Systems Limited | Scarborough | 2010 |
| ATD (NE) Ltd | Chester Le Street | 2016 |
| ATD Geoscience Ltd | Bristol | 2020 |
| ATE Land Solutions Limited | Telford | 2021 |
| Aten Petroleum Limited | London | 2020 |
| ATF Services (Moray) Limited | Elgin | 2023 |
| Athena Exploration Limited | Aberdeen | 2013 |
| Athena Inspection Ltd | Great Yarmouth | 2024 |
| Athena Systems Ltd | Dunfermline | 2025 |
| Atholl Technical Solutions Limited | Aberdeen | 2007 |
| Atinor Ltd | London | 2025 |
| ATJ Afhagrp Recovery Limited | Bracknell | 2017 |
| ATJ Recovery Scot Group Limited | Kinross | 2017 |
| Atlantic International Operation Limited | Edgware | 2005 |
| Atlantic Marine & Oilfield Services Ltd | Mildenhall | 2025 |
| Atlantic Petroleum North Sea Limited | London | 2007 |
| Atlantic Petroleum UK Limited | London | 2002 |
| Atlantic Plant Hire, Drilling, and Crushing Ltd | Dungannon | 2017 |
| Atlantis Well Engineering Limited | Dundee | 2025 |
| Atlas Energy Management Ltd | Ickenham | 2013 |
| Atlas GAS Heating and Plumbing Service Limited | Birmingham | 2021 |
| Atlas Lion Advisory Ltd | London | 2025 |
| Atlas Strategic Finance Limited | London | 2026 |
| Atmosphere Company for Environmental and Laboratory Services Ltd | Ruislip, | 2023 |
| Atomcraft Limited | Edzell | 2007 |
| Atrina Drilling Limited | Ormskirk | 2013 |
| Atticus Energy Ltd | Cannock | 2014 |
| Attock OIL Company Limited(The) | Croydon | 1913 |
| Attollo Ltd | London | 2014 |
| Atzusni Limited | Enniskillen | 2023 |
| AU Services Ltd | Leeds | 2015 |
| Auguste & Sons Ltd | Bromley | 2018 |
| AUK Energy Management Ltd | Aberdeen | 2006 |
| AUM International FZE Ltd | London | 2025 |
| AUM Invest Ltd | Saffron Walden | 2025 |
| Aurelia Mining Limited | London | 2021 |
| Aurelian OIL & GAS Limited | London | 1982 |
| Aurevia Resources Limited | Harlow | 2025 |
| Auriga Royalties Limited | London | 2025 |
| Auriix Ltd | London | 2020 |
| Aurora Energy Resources Limited | Aberdeen | 2008 |
| Aurora Gold Resources Limited | Bournemouth | 2025 |
| Aurum House Limited | Rotherham | 2015 |
| Aurum Mining Syndicate Ltd | London | 2025 |
| Aurum Recovery Limited | London | 2024 |
| Aurum Tech Minerals Holding Ltd | Cardiff | 2024 |
| Austia Limited | Epsom | 2017 |
| Australia Bondunte Biotechnology Co., Ltd | Cardiff | 2026 |
| Australia Olanwo Import and Export Group Co., Ltd | Cambridge | 2024 |
| Automation Control Systems Ltd | London | 2008 |
| AVC Systems Ltd | Blackpool | 2019 |
| Aventra Global Ltd | London | 2025 |
| Aventure Ltd | London | 2021 |
| Aventus AI Limited | Westhill | 2018 |
| Avenue 62 Limited | London | 2016 |
| Avery Inspection Services Limited | Norwich | 2002 |
| Avesoro Services (UK) Limited | London | 2011 |
| Avid Gold Ltd | London | 2026 |
| Aviera Global Consulting Ltd | London | 2024 |
| Avitar Services Limited | Billericay | 2001 |
| Avocet Resources Limited | Warrington | 2005 |
| Avon Material Supplies Limited | Wimborne | 1993 |
| AW Well Services Ltd | Lincoln | 2019 |
| Awau Ltd | Torthorwald | 2024 |
| Awcre Limited | Glasgow | 2017 |
| Awfabmech Ltd | Montrose | 2017 |
| Awka Limited | Whitby | 2013 |
| Awkj Limited | Whitby | 2012 |
| AWW Subsea Engineering Services Ltd | Cambuslang | 2015 |