| Johnson Controls Hvac Ltd | Prenton | 2026 |
| Johnson Utility Solutions Limited | Liverpool | 2016 |
| Johnsons Generators Limited | Chandlers Ford | 2015 |
| Johnston & Johnston Consulting Ltd | Prestbury | 2015 |
| Join Lucy Limited | Surrey | 2021 |
| Joju Charging Network Limited | London | 2018 |
| Joju Limited | London | 2006 |
| Joju Solar Limited | London | 2016 |
| Jolo Energy Limited | London | 2017 |
| Jolt Charge Limited | London | 2021 |
| Jones and Jones Renewables Ltd | Neath | 2013 |
| Jones Biomas Limited | Dudley | 2017 |
| Jones Distribution Services Ltd | Norwich | 2014 |
| Jonstar Energy Brokers Ltd | Leicester | 2015 |
| Joply Power Limited | London | 2011 |
| Jordan Energy Investment 2 Ltd | London | 2014 |
| Jordanston Solar Limited | Reading | 2011 |
| Joule Sustainable Energy Solutions Ltd | Worcester | 2021 |
| Joulium Ltd | London | 2025 |
| Joval Multi-Choice Limited | Mitcham | 2020 |
| Jovbag Limited | London | 2025 |
| Jozi Power Limited | Camberley | 2006 |
| JP Air-Conditioning Services Limited | Bromley | 2012 |
| JP Capital Partners Limited | Fareham | 2012 |
| JP Solar Ltd | Birmingham | 2025 |
| JP Wind Limited | London | 2011 |
| Jpak Lighting Limited | High Wycombe | 2016 |
| JPD Solar Solutions Ltd | Bracknell | 2023 |
| JPF Maintenance Services Ltd | Essex | 2017 |
| JPF Solutions Limited | Ossett | 2013 |
| JPH AC Limited | Gravesend | 2015 |
| JPK Electrical Contractors Limited | Sandhurst | 2012 |
| JPK Lighting Limited | High Wycombe | 2022 |
| JPM Autobodies Truck and Van Limited | North Shields | 2019 |
| JR Cooling Ltd | Derby | 2020 |
| JRB Air Conditioning Limited | Ascot | 2024 |
| JRC Cooling Ltd | Leeds | 2026 |
| JRH Electrical LC Limited | Cardiff | 2017 |
| JRL Realty Ltd | Falkirk | 2023 |
| JS Air Tech Ltd | Hull | 2025 |
| JS Energy Ventures Limited | Torquay | 2025 |
| JS Mechanical Building Services Ltd | Bromsgrove | 2020 |
| JS Wind Energy Ltd | Newcastle Upon Tyne | 2019 |
| JSC Air Conditioning Limited | Sandwich | 2013 |
| JSC Air Conditioning Solutions Limited | Birmingham | 2024 |
| JSC Aircon Services Ltd | Dover | 2018 |
| Jscsolars Ltd | Wolverhampton | 2024 |
| JSF Wind Energy Limited | Montrose | 2013 |
| Jshsolars Ltd | Southsea | 2025 |
| JSK Utilities Limited | Witham | 2017 |
| JSL Resourcing Limited | Chorley | 2018 |
| JSM Utility Solutions Ltd | Wareham | 2026 |
| JSR Energy Limited | Guildford | 2012 |
| JSR Solutions Ltd | Farnborough | 2021 |
| JT Construction Ltd | Stockton-On-Tees | 2010 |
| JT Energy Storage Ltd | Newcastle Upon Tyne | 2021 |
| JT3 Utilities Limited | Hatfield | 2020 |
| JTB Electrical Glos Limited | Dursley | 2020 |
| JTC Electrical Installation Ltd | Yateley | 2022 |
| Jtek Electrical Ltd | Norwich | 2015 |
| Juice Solar Ltd | Bolton | 2025 |
| Juicy Energy Group Ltd | London | 2025 |
| Jukes Electrical Limited | Dartford | 2014 |
| Jukes Partners Limited | Dartford | 2014 |
| Julian LME Electrical Company Ltd | Ilford | 2017 |
| Julien Wheel Ltd | London | 2022 |
| Juliens Property Maintenance Limited | Hayes | 2023 |
| Jumec Ltd | Birmingham | 2024 |
| Junin Energy Limited | London | 2017 |
| Juniper Energy Limited | Sunderland | 2024 |
| Juno 1 Limited | Shepton Mallet | 2025 |
| Juno 2 Limited | Shepton Mallet | 2022 |
| Jupiter Electrical Services Ltd | Bootle | 2025 |
| Jura Solar Limited | London | 2011 |
| Jurair Ltd | Coatbridge | 2025 |
| Just Air (Services) Limited | Cardiff | 2012 |
| Just B2B Utilities Ltd | Newport | 2023 |
| Just Chillin' Air Conditioning Ltd | Stroud | 2019 |
| Just Coolers Limited | Charlton Marshall | 2023 |
| Just Electricals Ltd | Dagenham | 2018 |
| Just Electrics Limited | Northamptonshire | 2002 |
| Just Energy Consulting Ltd | Edinburgh | 2018 |
| Justfarmenergy Limited | | 2006 |
| Justin Electrical Services Ltd | London | 2018 |
| Justus Energy Ltd | Kington | 2024 |
| Jute Dundee Limited | London | 2014 |
| Jute Holdings Limited | London | 2015 |
| JV Noble GAS & Heating Engineering Ltd | Chipping Norton | 2022 |
| JVD North Limited | Inverness | 2012 |
| JW Cooling Ltd | Edgware | 2023 |
| JW HV Ltd | Brighton | 2023 |
| JW Moore Utilities Ltd | Braintree | 2025 |
| JWM Electrical Services (Norfolk) Ltd | Swansea | 2012 |
| Jwparish Ltd | Houghton Le Spring | 2018 |
| K a D Contractors Limited | South Shields | 2016 |
| K G Technical Services (East Anglia) Ltd | Ipswich | 2016 |
| K J Air Ltd | Hyde | 2004 |
| K J Refrigeration Ltd | Barnstaple | 2003 |
| K Laidlaw Projects Ltd | Dundee | 2024 |
| K S Energy Solution Ltd | Glasgow | 2023 |