| J Wilks Mechanical Ltd | Wholesale & retail | 2021 |
| J&C Bespoke Cleaning Limited | Administrative & support | 2024 |
| J&C Carpentry Building Ltd | Manufacturing | 2025 |
| J&D Landscapes (Herts) Ltd | Administrative & support | 2020 |
| J&D Mechanical Services London Ltd | Construction | 2025 |
| J&D Waste Services Ltd | Water & waste | 2023 |
| J&K Cawdron Limited | Hospitality | 2017 |
| J&N Properties Cambs Ltd | Real estate | 2024 |
| J&R Systems Limited | Professional & scientific | 2025 |
| J&Ss Accountancy Limited | Professional & scientific | 2014 |
| J. a. Michell Engineering Limited | Manufacturing | 1973 |
| J. a. Robinson VET Services Ltd | Professional & scientific | 2023 |
| J. Ireland Scenery Ltd | Construction | 2018 |
| J. Miles Building Ltd | Construction | 2024 |
| J.A.K. Carpentry Ltd | Manufacturing | 2019 |
| J.Goldson Ltd | Information & communication | 2023 |
| J.M.J. Holdings Limited | Professional & scientific | 1986 |
| J.M.J. Precision Sheet Metal Limited | Manufacturing | 2003 |
| J.Mukasa Limited | Wholesale & retail | 2026 |
| J2JK UK Limited | Wholesale & retail | 2014 |
| Jabez COK Ltd | Real estate | 2025 |
| JAC Holdings (Hertfordshire) Ltd | Financial services | 2016 |
| JAC Holdings (Topco) Limited | Financial services | 2026 |
| Jack Joblin Rigging Ltd | Construction | 2025 |
| Jack Nicholas Finishing Limited | Other services | 2025 |
| Jack Nicholas Group Ltd | Construction | 2020 |
| Jack Nicholas Limited | Manufacturing | 2017 |
| Jack the Gripper Services Limited | Other services | 2022 |
| Jackson's Contractors Limited | Administrative & support | 2025 |
| Jackson's Demolition & Environmental Ltd | Construction | 2023 |
| Jackson's Yard LLP | | 2019 |
| Jacmol Property Solutions Ltd | Construction | 2026 |
| Jacob Dudman Limited | Arts & recreation | 2026 |
| Jacob Lunt Limited | Arts & recreation | 2003 |
| Jacqueline Wambui Mwaura Ltd | Wholesale & retail | 2021 |
| Jacy Media Group Ltd | Information & communication | 2019 |
| Jade Projects Limited | Wholesale & retail | 2015 |
| Jadecare Limited | Health & social care | 2024 |
| JAE Software Ltd | Information & communication | 2023 |
| Jaggi Plumber Limited | Construction | 2022 |
| Jain Care Ltd | Health & social care | 2006 |
| James and Partners Limited | Administrative & support | 2013 |
| James and the Giant Veitch Ltd | Arts & recreation | 2014 |
| James Coley Associates Limited | Professional & scientific | 2012 |
| James Estates (Anglian) Limited | Construction | 2022 |
| James Estates Limited | Real estate | 2015 |
| James Gerrard Consulting Ltd | Administrative & support | 2026 |
| James Judge Limited | Information & communication | 2003 |
| James Lavington Properties Limited | Real estate | 2012 |
| James Leisure Holdings Limited | Financial services | 2026 |
| James Property Investments LLP | | 2008 |
| James Richards CS Ltd | Wholesale & retail | 2021 |
| James Veale Jewellery Limited | Manufacturing | 2013 |
| Jane Street Limited | Information & communication | 2014 |
| January Pictures Limited | Information & communication | 2017 |
| Janus Sync Limited | Information & communication | 2024 |
| Jardak Services Limited | Construction | 2001 |
| Jarroxs Limited | Financial services | 2014 |
| JAS Property Ventures Ltd | Real estate | 2020 |
| Jashapa Ltd | Professional & scientific | 2019 |
| JAT Productions Ltd | Information & communication | 2023 |
| JAY BEE Laundry Services Limited | Other services | 2020 |
| JAY Malaxmi Limited | Wholesale & retail | 2026 |
| Jayden James Ltd | Professional & scientific | 2019 |
| Jayden Lewis Group Limited | Professional & scientific | 2021 |
| Jayosa Ltd | Health & social care | 2013 |
| JB Fire and Electrical Ltd | Construction | 2026 |
| JB Studio Services Ltd | Information & communication | 2019 |
| JBL Property Group (DIV 01) Limited | Real estate | 2016 |
| JBL Property Group (DIV 02) Limited | Real estate | 2016 |
| JBL Property Group (DIV 03) Limited | Real estate | 2016 |
| JBL Property Group (DIV 04) Limited | Real estate | 2016 |
| JBL Property Group (DIV 05) Limited | Real estate | 2018 |
| JBL Property Group (DIV 06) Limited | Real estate | 2018 |
| JBL Property Group (DIV 07) Limited | Real estate | 2018 |
| JBL Property Group (DIV 08) Limited | Real estate | 2020 |
| JBL Property Group (DIV 09) Limited | Other services | 2020 |
| JBL Property Group (DIV 10) Limited | Real estate | 2021 |
| JBL Property Group Investments LLP | | 2023 |
| JBL Property Group Limited | Other services | 2016 |
| JBL Property Group Services Limited | Real estate | 2019 |
| Jburns Prime Trading Limited | Administrative & support | 2026 |
| JC Fitness Performance Limited | Health & social care | 2026 |
| JC Investment Management Ltd | Administrative & support | 2011 |
| JC Removal & Repair Ltd | Construction | 2025 |
| Jcab Medical Limited | Health & social care | 2025 |
| Jcleanways Ltd | Administrative & support | 2023 |
| Jdclayton Capital Limited | Other services | 2020 |
| Jdclayton Holdings Limited | Financial services | 2020 |
| Jdel Solutions Limited | Real estate | 2017 |
| JDL Music Limited | Information & communication | 2025 |
| JDM IT Systems Consultants Ltd | Extraterritorial | 2018 |
| JE Signs Ltd | Manufacturing | 2023 |
| Jeakioe Limited | Health & social care | 2016 |
| Jeamel Publishing Limited | Information & communication | 2014 |
| Jechi Enterprises Limited | Health & social care | 2015 |
| Jeelzo Ltd | Wholesale & retail | 2025 |
| Jeepster Ltd | Wholesale & retail | 2020 |
| Jeer Holdings Limited | Professional & scientific | 2023 |
| Jeeves Yard Residents Limited | Households | 2002 |