| James Farrelly Limited | Other services | 2015 |
| James Oak & Co Ltd | Wholesale & retail | 2026 |
| Janpraci Fine Jewellery Co., Ltd | Extraterritorial | 2021 |
| JAR Records Limited | Information & communication | 2009 |
| JAR Songs Limited | Information & communication | 2010 |
| Jarratec Limited | Agriculture & fishing | 1978 |
| Jasper & Tallow Ltd | Wholesale & retail | 2016 |
| Jazzbomb Events Limited | Arts & recreation | 2008 |
| JBI Supplies Ltd | Wholesale & retail | 2019 |
| JBK Consulting Limited | Professional & scientific | 2019 |
| JBR Contracts Ltd | Construction | 2013 |
| JBT Properties Ltd | Real estate | 2016 |
| Jbyron Design Ltd | Administrative & support | 2017 |
| JCC Components Limited | Wholesale & retail | 2025 |
| Jcds Consultancy Limited | Information & communication | 2013 |
| Jcmft Ltd | Other services | 2019 |
| JDA Services Ltd | Information & communication | 2007 |
| JDD Kent Limited | Households | 2024 |
| JDE Properties Limited | Construction | 2006 |
| JDV Projects Ltd | Professional & scientific | 2017 |
| Jenco Limited | Administrative & support | 2013 |
| Jenson Property Services Limited | Construction | 2016 |
| Jeremy LEE Limited | Hospitality | 2012 |
| Jewel of Hearts Limited | Wholesale & retail | 2012 |
| JF Services & Contracting Ltd | Construction | 2025 |
| JID Consultancy Limited | Professional & scientific | 2019 |
| Jigsaw Performing Arts Ealing Limited | Arts & recreation | 2025 |
| JIN XIN Cater Ltd | Hospitality | 2020 |
| Jinzan Trading Co,. Ltd | Extraterritorial | 2021 |
| JJF Management Limited | Professional & scientific | 2018 |
| Jjfhm LLP | | 2023 |
| JJH Music Ltd | Information & communication | 2019 |
| JJS Global Developments Limited | Professional & scientific | 2024 |
| Jjwcivils Ltd | Construction | 2021 |
| JK Fullhouse Developments Ltd | Construction | 2026 |
| JK Fullhouse Property Ltd | Households | 2024 |
| JKL Business Holdings Limited | Financial services | 2009 |
| JKL Solutions Limited | Wholesale & retail | 2009 |
| JKN Electrical Limited | Construction | 2020 |
| JL & Sons Haulage Ltd | Transport & logistics | 2015 |
| JL Sales Ltd | Wholesale & retail | 2021 |
| JLB Construction Limited | Construction | 2026 |
| JLH Innovations Ltd | Health & social care | 2019 |
| JLI Management Limited | Administrative & support | 2018 |
| JLJ Property Group & Associates Ltd | Real estate | 2024 |
| JLS (Harlow) Ltd | Financial services | 2020 |
| JM Blinds Ltd | Other services | 2021 |
| JMC Management Limited | Information & communication | 2006 |
| JMC Management SPV Ltd | Real estate | 2018 |
| JMJ Accountancy Ltd | Professional & scientific | 2012 |
| Jmriverscapes Ltd | Professional & scientific | 2025 |
| JNH Decorators Limited | Construction | 2010 |
| JNL Professionals Limited | Real estate | 2025 |
| Joannou's Renovation & Maintenance Ltd | Construction | 2025 |
| JOB Search AI Ltd | Administrative & support | 2025 |
| John Barber (Hertford) Limited | Real estate | 1934 |
| John Gargan Limited | Other services | 2013 |
| John Owen Construction Limited | | 1987 |
| John Smith-Butler and Sons Limited | Wholesale & retail | 1979 |
| Johngar Properties Ltd | Real estate | 2018 |
| Johnson & Co Estates Limited | Real estate | 2019 |
| Jojo's Cards & Gifts Limited | Wholesale & retail | 2017 |
| Jolana Holdings Limited | Financial services | 2025 |
| Jolly Partnership Limited | Professional & scientific | 2015 |
| JON Durbin Associates Limited | Professional & scientific | 2014 |
| Jonathan Bell Ltd | Health & social care | 2009 |
| Jones Electrical Contractors Ltd | Construction | 2015 |
| Joseph Crowley Limited | Professional & scientific | 2016 |
| Joseph Gibson Garden Design Limited | Administrative & support | 2018 |
| Joseph’S Teaching Services Limited | Education | 2022 |
| JP Aesthetics Limited | Health & social care | 2025 |
| JPC Developments (Ware) Limited | Real estate | 2003 |
| JPO Logistics Ltd | Transport & logistics | 2019 |
| JR Construction & Maintenance Ltd | Construction | 2025 |
| JR Residential Properties Limited | Real estate | 2016 |
| JS Retailers PVT Ltd | Wholesale & retail | 2018 |
| JSC Consultancy Limited | Construction | 2000 |
| JSG Real Estate Ltd | Real estate | 2017 |
| Jtcls Limited | Professional & scientific | 2014 |
| Judge Commodities Limited | Wholesale & retail | 2023 |
| Judkins Legal Consultancy Limited | Professional & scientific | 2024 |
| Juntara Thai Spa Ltd | Other services | 2020 |
| Jurio Luti Ltd | Wholesale & retail | 2025 |
| Just Bike Repairs Ltd | Wholesale & retail | 2025 |
| JW Electrical Solutions Limited | Construction | 2010 |
| JW Garden Rooms Limited | Construction | 2023 |
| K O Rowley Promotions Ltd | Real estate | 2018 |
| K+S UK & Eire Ltd. | Wholesale & retail | 1939 |
| K. D. Contractors Ltd | Construction | 2013 |
| K. Marsh MBE Consultancy Ltd | Professional & scientific | 2023 |
| K.W. Maintain & Secure Ltd | Construction | 2015 |
| K1 Berkshire Ltd | Arts & recreation | 2022 |
| K73CONSULTING Ltd | Professional & scientific | 2025 |
| K9 Festivals Ltd | Administrative & support | 2024 |
| K999 Animal Services Ltd | Professional & scientific | 2016 |
| KA Community Care Limited | Health & social care | 2022 |
| KAG Logistics Ltd | Transport & logistics | 2013 |
| Kaleidovision Limited | Information & communication | 1992 |
| Kape Fashion Ltd | Wholesale & retail | 2014 |
| Kara James International Ltd | Real estate | 2024 |