| Imperial Commander Ltd | London | 2020 |
| Imperial GAS & Power Ltd | Eastbourne | 2021 |
| Imperium Electrics Ltd | Bath | 2019 |
| Imperium Energy Group Ltd | London | 2025 |
| Import & Export Solar System & Distribution Ltd | Swanscombe | 2025 |
| Improved Energy Ltd | London | 2021 |
| Improved Services Limited | Tyne & Wear | 2004 |
| Impulse Cable Services Ltd | Dunfermline | 2024 |
| Impulse Energy Solutions (Wellingborough) Limited | Witney | 2017 |
| Impulse Energy Solutions Limited | Witney | 2013 |
| Imsap Services Limited | Campbeltown | 2018 |
| IN2GAS Ltd | Goole | 2015 |
| Ince BIO Power Limited | London | 2015 |
| Inchclett WF Limited | Motherwell | 2013 |
| Inchdairnie Renewable Energy Limited | London | 2016 |
| Independent Community Heating Limited | Bury St. Edmunds | 2005 |
| Independent Distribution Connection Specialists Limited | London | 2022 |
| Independent E-Nrg Limited | Liverpool | 2014 |
| Independent Electrical Ltd | Leeds | 2020 |
| Independent Pipelines Limited | Bury St. Edmunds | 1993 |
| Independent Power Connections Limited | Chesterfield | 2019 |
| Independent Power Networks Limited | Bury St. Edmunds | 2003 |
| Independent Power Operations Limited | London | 2001 |
| Indevo Energy Ltd | London | 2024 |
| Indian Q Storage Limited | London | 2022 |
| Indian Queens Power Limited | Hull | 1994 |
| Indieair Limited | Newmarket | 2026 |
| Indigo Generation Limited | London | 2016 |
| Indigo Pipelines Holdco 1 Limited | Reading | 2014 |
| Indigo Pipelines Holdco 2 Limited | Reading | 2014 |
| Indigo Pipelines Limited | Reading | 1992 |
| Indigo Power Limited | Reading | 2019 |
| Indigo Switch Ltd | Huddersfield | 2024 |
| Indigodice Ltd | Aberdeen | 2024 |
| Indigotigre Limited | Leicester | 2014 |
| Indiquip Ltd | Lisburn | 2024 |
| Individual Cooling Limited | Dagenham | 2018 |
| Inducta Energy Ltd | London | 2026 |
| Inducta Grid Ltd | London | 2026 |
| Inducta Infrastructure Ltd | London | 2026 |
| Inducta Power Ltd | London | 2026 |
| Industria Resources U.K. Limited | Gateshead | 2017 |
| Industria Solar Fallaws Limited | Chester Le Street | 2021 |
| Industria Solar Financials Limited | Chester Le Street | 2021 |
| Industria Solar Forfar Limited | Chester Le Street | 2021 |
| Industria Solar Limited | Chester Le Street | 2021 |
| Industria Solar Vigo Limited | Chester Le Street | 2021 |
| Industrial & Community Wind Generation Limited | Middlesbrough | 2010 |
| Industrial & Community Wind Generation NO. 2 Limited | Middlesbrough | 2012 |
| Industrial Power Limited | Brentwood | 2016 |
| Industrial Processes Automation Services Ltd | Thornton-Cleveleys | 2021 |
| Indygen Limited | Oldham | 2015 |
| Ineos Energy Trading and Marketing Holdings Limited | Lyndhurst | 2024 |
| Ineos Energy Trading Limited | Lyndhurst | 2017 |
| Ineos Infrastructure (Grangemouth) Limited | Lyndhurst | 2009 |
| Ineos Runcorn (TPS) Holdings Limited | Runcorn | 2008 |
| Ineos Runcorn (TPS) Limited | Runcorn | 2008 |
| Inevitable Energy Limited | Ruislip | 2025 |
| INF NRJ Limited | London | 2020 |
| Infinis (Ford Oaks) Limited | Northampton | 2022 |
| Infinis (Gowerton) Limited | Northampton | 2022 |
| Infinis (Re-Gen) Limited | Northampton | 1918 |
| Infinis (Shoreside) Limited | Northampton | 2021 |
| Infinis Alternative Energies Limited | Northampton | 2013 |
| Infinis Energy Storage Limited | Northampton | 2008 |
| Infinis Limited | Northampton | 2006 |
| Infinis Solar Limited | Northampton Business Park | 2014 |
| Infinite Biomass Ltd | Bridgend | 2014 |
| Infinite Data Limited | Bridgend | 2026 |
| Infinite FM Ltd | London | 2018 |
| Infinite Investments (Tafarnaubach) Ltd | London | 2014 |
| Infinite Pencarreg Limited | Bridgend | 2025 |
| Infinite Power Company Limited | Moor Row | 2018 |
| Infinite Renewable Solutions Ltd | Bridgend | 2013 |
| Infinite Renewables Group Limited | Bridgend | 2015 |
| Infinite Renewables Group Services Ltd | Bridgend | 2018 |
| Infinite Renewables Ltd | Bridgend | 2010 |
| Infinite Renewables Wales Ltd | Bridgend | 2023 |
| Infinite Solar Ltd | Bridgend | 2014 |
| Infinite Ventures (Blaen Morlais) Limited | Bridgend | 2013 |
| Infinite Ventures (Blaencilgoed) Ltd | London | 2012 |
| Infinite Ventures (Crican) Ltd | London | 2012 |
| Infinite Ventures (Ffyndaff) Limited | Bridgend | 2013 |
| Infinite Ventures (Goathill) Ltd | London | 2012 |
| Infinite Ventures (Haregrove) Limited | Bridgend | 2014 |
| Infinite Ventures (Llethr DDU) Limited | Bridgend | 2013 |
| Infinite Ventures (Nantycaws) Ltd | London | 2011 |
| Infinite Ventures (Redlands) Limited | London | 2011 |
| Infinite Ventures (Rexon) Ltd | London | 2013 |
| Infinite Ventures (Scoveston) Ltd | London | 2012 |
| Infinite Ventures (Shawell) Ltd | Bridgend | 2012 |
| Infinite Ventures (Waldrons) Ltd | Bridgend | 2012 |
| Infinite Ventures (Wentloog) Limited | Bridgend | 2013 |
| Infinity Air Conditioning Ltd | Flitwick | 2025 |
| Infinity Electrical Services Ltd | Bristol | 2014 |
| Infinity Nautical Services Limited | London | 2012 |
| Infinity Solar UK Limited | Wembley | 2015 |
| Infinity Utilities Solutions Limited | Slough | 2025 |
| Infra Balance Group Ltd | London | 2020 |
| Infra Balance New Energy Limited | London | 2021 |