| J & L Renewables Limited | Ludlow | 2013 |
| J & M Windpower Limited | Launceston | 2012 |
| J & R Renewable Ltd | Coventry | 2022 |
| J a Electrical Installations Ltd | Clacton-On-Sea | 2020 |
| J Aldridge Utilities Limited | Burnham-On-Crouch | 2018 |
| J and E Electrical Limited | W Yorks | 1999 |
| J B Bentley Holdings Limited | Cwmbran | 2020 |
| J Carlisle Ltd | Ballynahinch | 2025 |
| J Cotter Limited | Bromley | 2015 |
| J Coulson Wind Ltd | Newcastle Upon Tyne | 2021 |
| J D MEE & Sons Renewables Limited | Canvey Island | 2012 |
| J E C Sparks Ltd | Chislehurst | 2017 |
| J G C Engineering Limited | Bexley | 2017 |
| J G Pears Power Limited | Sheffield | 2016 |
| J H Cooling Solutions Ltd | Dromore | 2026 |
| J Hingley Utilities Ltd | Bromsgrove | 2026 |
| J J Bensons Electrical Ltd | Ongar | 2010 |
| J J Black Ltd | Tarbert | 2003 |
| J J Power Limited | Cirencester | 2009 |
| J J R Utilities Limited | Southend-On-Sea | 2024 |
| J J Services Ltd | Norwich | 2016 |
| J James Utilities Limited | Chandler'S Ford | 2016 |
| J L Frost & SON Wind Farm Limited | Watford | 2015 |
| J M Electrical (Keighley) Limited | Keighley | 2011 |
| J Nelson Renewables Ltd | Blyth | 2021 |
| J P Baldwin Electrical Ltd | Bingley | 2017 |
| J Prangle Ltd | Chippenham | 2009 |
| J R Energy Limited | Glasgow | 2012 |
| J Rivers Utilities Limited | Chelmsford | 2017 |
| J Sparks Electrical Ltd | Dartford | 2017 |
| J W Renewables Ltd | Peterlee | 2012 |
| J Walton Ltd | Morpeth | 2019 |
| J&M Electrical Services Ltd | London | 2017 |
| J&R Services (Essex) Ltd | Braintree | 2017 |
| J. C. Hydro Ltd | Derby | 2008 |
| J.Boulton Plumbing & Heating Ltd | Scunthorpe | 2024 |
| J.D. Fire Services Limited | Maidstone | 2009 |
| J.Fleming Engineering(Stornoway)Limited | Paisley | 1976 |
| J.S.Starnes & Sons Limited | London | 1944 |
| JA Mechanical Building Services Ltd | Bromsgrove | 2025 |
| JA Offshore Limited | Barrow-In-Furness | 2017 |
| JAB Power Ltd | Belfast | 2016 |
| Jabbah Bess Limited | Bristol | 2022 |
| Jablades Ltd | Shanklin | 2023 |
| Jace Air Conditioning Limited | High Wycombe | 2020 |
| Jack Trattles Electrical Limited | Newcastle Upon Tyne | 2016 |
| Jacks Lane Energy Limited | Leeds | 2013 |
| Jacob Developments Limited | Greater Manchester | 2010 |
| Jacobs Electrical Services Ltd | Gravesend | 2014 |
| Jadi Limited | Cookstown | 2017 |
| Jado Insaat Ltd | London | 2026 |
| Jafa Solar Farm Limited | London | 2021 |
| JAG Ecoenergy Ltd | Carmarthen | 2014 |
| Jagar Enterprises Limited | Slough | 2012 |
| Jagos Energy Limited | London | 2017 |
| JAI Shambho Trading Co., Ltd | London | 2023 |
| Jake Uttley Electrical Limited | Mexborough | 2025 |
| JAM Solar Limited | London | 2025 |
| Jamech (UK) Limited | Brentwood | 2014 |
| James Moore Electrical Limited | Cullompton | 2011 |
| James Munro (Electrical) Limited | Glasgow | 1974 |
| James Steele Ltd | Lanark | 2012 |
| James@Parker Ltd | Bradford | 2021 |
| Jameson Road Energy Limited | London | 1999 |
| Jamm Power Limited | Turriff | 2014 |
| Jams Electrical Ltd | Gravesend | 2019 |
| JAN Bezemer & Sons Limited | Middlesbrough | 2002 |
| Janeric Ltd | Turriff | 2015 |
| Janneys GAS Services Limited | Washington | 2026 |
| Janoel Electrical Limited | Epsom | 2018 |
| Jarjouratech Ltd | London | 2021 |
| Jarvis-Williams Electrical & Data Services Limited | Braintree | 2017 |
| Javan Energy Limited | London | 2017 |
| JAY CON AC Ltd | Coventry | 2022 |
| JAY Mackie & Sons Ltd | Seaford | 2017 |
| Jayden Electrical Ltd | Sudbury | 2015 |
| Jaydowe Limited | Wimborne | 2021 |
| Jayes Energy Ltd | Dorking | 2011 |
| Jaylee Refrigeration Ltd. | Dorchester | 2003 |
| Jaylow Supplies Limited | London | 1949 |
| JB Airconditioning Services Ltd | Longfield | 2005 |
| JB Cooling Services Ltd | Chesterfield | 2017 |
| JB Electrical & Mechanical Services Ltd | Renfrew | 2010 |
| JB Lighting Ltd | Colindale | 2014 |
| Jb&D Electrical Ltd | Croydon | 2023 |
| JBK Insulation Ltd | Hartlepool | 2020 |
| JBK Project Limited | Leicester | 2013 |
| JBL Electrical Ltd | Brentwood | 2023 |
| Jblb Plumbing & Heating Services Ltd | Dundee | 2020 |
| JBM Solar Projects 18 Ltd | London | 2019 |
| JBM Solar Projects 2 Ltd | Swindon | 2019 |
| JBM Solar Projects 23 Limited | London | 2020 |
| JBM Solar Projects 6 Ltd | Swindon | 2019 |
| JBR Contractors Ltd | Hounslow | 2017 |
| JBS Electrics Ltd | London | 2015 |
| JC Air Conditioning Ltd | Upminster | 2020 |
| JCB GAS Heating Services Ltd. | Milton Keynes | 2014 |
| Jcblythin Ltd | Llandudno | 2023 |
| JCE Refrigeration Ltd | Wakefield | 2003 |
| JCF Developments (Yorkshire) Ltd | Bradford | 2024 |