| Intrinsic New Energy Ltd | London | 2026 |
| Intuitive Utility Solutions Ltd | London | 2026 |
| Intwood Bess Ltd | London | 2023 |
| Invenergy Development UK Ltd | Edinburgh | 2022 |
| Inver Burn Holdings Ltd | Maghera | 2024 |
| Inver Hydro Limited | Lochgilphead | 2016 |
| Inver Renewables Limited | Port Glasgow | 2014 |
| Inverbroom Hydro Limited | London | 2018 |
| Inverclyde Windfarm Limited | Larbert | 2011 |
| Inverliever Hydro Limited | Lanark | 2007 |
| Invernoaden Limited | London | 2008 |
| Inverra Holdings Ltd | London | 2025 |
| Invervar Hydro Development Limited | Perth | 2013 |
| Invicta Hydrogen Systems Limited | Birmingham | 2017 |
| Invictus Energy Consultants Limited | Chelmsford | 2021 |
| Invictus Services Group Ltd | Derby | 2026 |
| Iogen New Energies UK Ltd | Kingston Upon Thames | 2026 |
| Iona Electrics Limited | Newry | 2013 |
| Ions-Net Limited | Hook | 2022 |
| IOW Solar Limited | London | 2013 |
| IPC Grid Connection Limited | London | 2023 |
| IPC Grid Connection Two Limited | London | 2023 |
| IPC New World Energy Limited | London | 2023 |
| Ipeh Limited | Dromore | 2011 |
| IPG Integrated Power PLC | London | 1999 |
| IPM Energy Retail Limited | 1 Oxford St | 2007 |
| IPM Enterprises Ltd | Maidstone | 2017 |
| Ipower Flexible Energy Ltd | Lisburn | 2009 |
| Ipower Smart Energy Cluster Ltd | London | 2025 |
| Ipowerconsultancy Limited | London | 2026 |
| IPS Power Limited | Kingsbridge | 2009 |
| Ipsa Group PLC | Preston | 2005 |
| IQ Energy Centre Limited | Essex | 2018 |
| Iqony Renewables UK Ltd | Peterborough | 2018 |
| IR DNO Colbrans Holdco Limited | Cheltenham | 2021 |
| IR DNO ESS Limited | Cheltenham | 2023 |
| IR DNO Gowhole Holdco Limited | Cheltenham | 2021 |
| IR DNO Limited | Cheltenham | 2022 |
| IR12 Limited | Cheltenham | 2022 |
| IR14 Limited | Cheltenham | 2022 |
| IRE 001 Limited | Exeter | 2021 |
| Irena Bess Limited | Bristol | 2022 |
| Irene Power Company Limited | Chorleywood | 2006 |
| Irenka Limited | Fraserburgh | 2008 |
| Iresa Limited | Birmingham | 2012 |
| Irish Hill Wind Ltd | Belfast | 2016 |
| Iron Acton Green Limited | London | 2020 |
| Ironbridge Bess Limited | Cheltenham | 2022 |
| Ironbridge Energy Limited | London | 2023 |
| Ironhide Generation Limited | London | 2015 |
| Ironstone Energy Limited | Guildford | 2022 |
| Irvine Bay Events Limited | Glasgow | 2011 |
| Irvine Hydrogen Limited | Roslin | 2026 |
| Irwell Power Limited | Bristol | 2016 |
| Isafe Homes Ltd | Motherwell | 2020 |
| Isam Global Links UK Ltd | Manchester | 2025 |
| Iscold Automotive Ltd | Ulceby | 2019 |
| Isenau Projects Group Services Limited | London | 2023 |
| Iseo Solar Limited | London | 2022 |
| ISG Renewables Limited | Cheltenham | 2022 |
| Isho Studio Ltd | London | 2020 |
| Iskraemeco (UK) Limited | Horsham | 1995 |
| Island Air Conditioning Limited | Newport | 2006 |
| Island Cooling Ltd | Benfleet | 2022 |
| Island Energies Limited | London | 2020 |
| Island Energy Ltd | Craigavon | 2026 |
| Island GAS Operations Limited | Lincoln | 2000 |
| Island Power Solutions Ltd | London | 2018 |
| Islandmagee Energy Hub Limited | Larne | 2018 |
| Islandmagee Energy Limited | Belfast | 2006 |
| Islay Energy Trust | Bowmore | 2005 |
| Islay Renewable Energies Ltd | Bowmore | 2007 |
| Islay SM Limited | London | 2016 |
| Isle of Muck Power Limited | Mallaig | 1998 |
| Isle of Wight Grange Limited | Hove | 2013 |
| Isle of Wight Solar C.I.C. | London | 2019 |
| Isles of Scilly Community Venture C.I.C. | Isles Of Scilly | 2017 |
| Islip Road Renewable Energy Limited | Reading | 2025 |
| Islip Solar Park Limited | Exeter | 2011 |
| ISM Electrical Ltd | Harrow | 2022 |
| ISP (UK) 1 Limited | Gatwick | 2012 |
| Israsha Fuel and IT Limited | Ilford | 2025 |
| Italy Solar Generation One Limited | Bristol | 2018 |
| Itchen Power Limited | Warrington | 2014 |
| Itps Renewables Energy Ltd | Weymouth | 2008 |
| Iver Bess Ltd | London | 2023 |
| IVL IRE 001 Ltd | Ballymena | 2023 |
| IVY Air Conditioning Ltd | Shefford | 2025 |
| IWK Energy Ltd | London | 2026 |
| Iwmcontracts Ltd | Halstead | 2025 |
| Ixworth Solar Farming Limited | Attleborough | 2012 |
| Iykonie Ltd | London | 2022 |
| IZ Connect Limited | Guildford | 2019 |
| J & a Energy Limited | Stranraer | 2011 |
| J & a Renewables Limited | Draperstown | 2015 |
| J & AMV Ltd | Dungiven | 2020 |
| J & B Green Energy Ltd | Bexley | 2012 |
| J & G Engineering Contractors Limited | York | 2010 |
| J & H Whiteford Limited | Carluke | 2011 |
| J & J Energies Ltd | Bushmills | 2013 |