| K&T Electromech Services Limited | Orpington | 2020 |
| K-Nev Air Conditioning Ltd | New Mills | 2023 |
| K-Tec Wind Ltd | Montrose | 2025 |
| K.J. Howley (Electrical) Limited | Dewsbury | 2003 |
| K2 GAS Plumbing & Heating Limited | Bromley | 2025 |
| K2 Power Ltd | Ballyclare | 2015 |
| K2 Renewable Energy Ltd | Leeds | 2022 |
| KAB Electrical Ltd | London | 2020 |
| Kablepulse Ltd | Newcastle Upon Tyne | 2025 |
| KAF Electrical Ltd | Reading | 2017 |
| KAG Wiring Services Limited | Lowestoft | 2014 |
| Kagend Ltd | Norwich | 2011 |
| Kagu Energy Limited | London | 2017 |
| KAI Power Ltd | Glasgow | 2025 |
| Kala Power Limited | Bristol | 2010 |
| Kali Electrics Ltd | London | 2025 |
| Kali Energy Ltd | London | 2025 |
| Kaltun UK Limited | London | 2026 |
| Kaman Energy Limited | London | 2021 |
| Kamms Heating & Plumbing Ltd | Cardiff | 2019 |
| Kanadevia Inova Biogas Gravel PIT Ltd | London | 2015 |
| Kanadevia Inova Biogas Home Farm Ltd | York | 2013 |
| Kanadevia Inova Biogas Howla HAY Ltd | York | 2012 |
| Kanadevia Inova Biogas Leeming Limited | London | 2014 |
| Kanadevia Inova Biogas Lower Drayton Limited | London | 2017 |
| Kanadevia Inova Biogas ST Boswells Ltd | London | 2014 |
| Kanadevia Inova Biogas Wardley Limited | London | 2018 |
| Kanadevia Inova Biogas Washfold Farm Ltd | York | 2013 |
| Kanadevia Inova Biogas Westholme Farm Ltd | York | 2013 |
| Kanadevia Inova Biogas Wray House Ltd | York | 2014 |
| Kanay Commercial Ltd | Cardiff | 2022 |
| Kappa Solar Limited | London | 2013 |
| Kappacity Ltd | London | 2024 |
| Karby Michael Ltd. | Bishop'S Stortford | 2014 |
| Kari Energy Limited | London | 2020 |
| Karilin Energy Systems Ltd | London | 2025 |
| Karm Energy Holding Ltd | London | 2025 |
| Karpathos Energy Limited | London | 2017 |
| Karpowership Latam Limited | Knutsford | 2016 |
| Karpowership UK Limited | Knutsford | 2016 |
| KAT1 GAS Safety Services Limited | Liverpool | 2014 |
| Katofizzy Ltd | Belfast | 2026 |
| Kavanagh's a C Services Limited | Bilston | 2024 |
| Kaveh Solar Ltd | Bristol | 2022 |
| Kaya Refrigeration and Air Conditioning Supplies Limited | Greenford | 2025 |
| Kazakh Independent Power Limited | London | 2019 |
| KB Cooling Limited | Bolton | 2022 |
| KBC Innovations Ltd | Manchester | 2024 |
| KCCRAWLEY83 Limited | Whitby | 2018 |
| KD 6A Limited | Harlow | 2020 |
| KD Utilities Limited | Newton Aycliffe | 2013 |
| KD6B Limited | Harlow | 2021 |
| Keadby Developments Limited | Scunthorpe | 1992 |
| Keadby Generation Limited | Scunthorpe | 1992 |
| Keadby Next Generation Limited | Reading | 2024 |
| Kearsley Bess Limited | Cheltenham | 2022 |
| Keekle Power Limited | London | 2015 |
| Keep Cool Limited | Haywards Heath | 2022 |
| Keep Energy Contracts Limited | Widnes | 2019 |
| Keep Energy M & E Limited | Widnes | 2019 |
| Keeranbeg Wind Ltd | Cookstown | 2014 |
| Kegworth Energy Storage Park Limited | London | 2023 |
| Keigen Ltd | Swanley | 2019 |
| Keith Energy Storage Park Limited | London | 2024 |
| Keith Solar PV Limited | London | 2022 |
| Keithfield Energy Limited | Ellon | 2008 |
| Keithick Biogas Limited | London | 2013 |
| Keithick Energy Ltd | Perthshire | 2012 |
| Kelburn Hydro Limited | Glasgow | 2014 |
| Kelham Central Utilities Ltd | Bradford | 2021 |
| Kellas Drum Limited | Edinburgh | 2022 |
| Kellenergy Limited | Dungannon | 2016 |
| Kellingley Solar Farm Limited | Gatwick | 2015 |
| Kells BES Ltd | Belfast | 2018 |
| Kells Solar PV Ltd | Weybridge | 2014 |
| Kelly Green Limited | Bodmin | 2012 |
| Kelly Wind Energy Ltd | Enniskillen | 2012 |
| Kelmarsh Wind Farm Limited | London | 2014 |
| Kelp Systems Ltd | Edinburgh | 2021 |
| Kelroda Power Ltd | Fraserburgh | 2025 |
| Kelsick House (AD) Limited | Carlisle | 2013 |
| Keltneyburn Hydro Ltd | Perth | 2002 |
| Kemberton Solar Limited | London | 2020 |
| Kempstone Hill Wind Energy Limited | Aberdeen | 2010 |
| Kemsley CHP Limited | Nottingham | 2016 |
| Kencot Hill Solar Farm Limited | London | 2012 |
| Kenley House Solar Limited | Arundel | 2022 |
| Kenn Bros | Ellesmere | 2025 |
| Kennergy Ltd | Dunbar | 2013 |
| Kenninghall Solar Ltd | Norwich | 2019 |
| Kensus Ltd | Birmingham | 2024 |
| Kent Air Solutions Limited | Chatham | 2018 |
| Kent Cabling Networks Ltd | Rochester | 2009 |
| Kent Climate Solutions Ltd | Herne Bay | 2020 |
| Kent Community Energy Projects 1 CIC | London | 2025 |
| Kent Cooling Solutions Limited | Sidcup | 2013 |
| Kent Eco Energy Ltd | Maidstone | 2023 |
| Kent GAS Boiler Services Limited | Bromley | 2007 |
| Kent Group Services Limited | Walsall | 1994 |
| Kent Heating Solutions Ltd | Edenbridge | 2013 |