| Ithaca Exploration Limited | Leeds | 2006 |
| Ithaca Gamma Limited | Leeds | 2006 |
| Ithaca J E&P Limited | Leeds | 2014 |
| Ithaca MA Limited | Leeds | 2000 |
| Ithaca Ma(Ns) Limited | Aberdeen | 2002 |
| Ithaca Minerals (North Sea) Limited | Aberdeen | 2004 |
| Ithaca OIL and GAS Limited | Leeds | 1981 |
| Ithaca Petroleum Limited | Leeds | 2004 |
| Ithaca SP E&P Limited | Leeds | 1980 |
| Ithaca Ukcs Limited | Leeds | 1971 |
| Ithaca Zeta Limited | Leeds | 2014 |
| Itinerant Resources Public Limited Company | London | 1996 |
| Itog Services Limited | Lowestoft | 2025 |
| ITP Electrical Limited | Newquay | 2010 |
| Itsinspection Ltd | Peterhead | 2025 |
| Ivors Holdings Company and Consultancy Ltd | London | 2021 |
| Ivory Petroleum Ltd | Greenhithe | 2025 |
| IWC Interventions Limited | Aberdeen | 2007 |
| Iwcf Operations Limited | Montrose | 2017 |
| IYF Solutions Limited | Aberdeen | 2021 |
| Izomax Limited | Aberdeen | 2025 |
| J & a Plant Hire Limited | Hope Valley | 2002 |
| J & D Services (Scotland) Ltd. | Ayr | 2005 |
| J & K Transport(Uk) Limited | Coventry | 2011 |
| J & L Access Ltd | Hebburn | 2026 |
| J & L Coppard Minex Limited | Rugby | 2016 |
| J & L Training Solutions Limited | Montrose | 2013 |
| J & R Cherry Limited | Dunvant | 2020 |
| J & V Kelly Limited | Solihull | 1997 |
| J a Chapman Ltd | Liverpool | 2025 |
| J a Flint Ltd | Munlochy | 2023 |
| J a Jemmeson Ltd | Darlington | 2023 |
| J a Mackay Services Ltd | Thurso | 2012 |
| J C Offshore Management Limited | Castle Douglas | 2011 |
| J Carter Renewables Ltd. | Stockton On Tees | 2022 |
| J E D Crushing & Screening Limited | New Tredegar | 2008 |
| J H Offshore Ltd | Scarborough | 2022 |
| J Haghkar Engineering Limited | Ashington | 2015 |
| J Harvey Offshore Services Ltd | Sittingbourne | 2018 |
| J Hopkins Ltd | Hebden Bridge | 2011 |
| J K S - Boyles UK Limited | Mansfield | 2001 |
| J Martinson Grab Lorry Hire Ltd | Garthorpe | 2019 |
| J Mccourt & SON Ltd | Belfast | 2019 |
| J O'Regan & Sons Limited | London | 2003 |
| J P Kenny Resources Ltd | Cardiff | 2013 |
| J P Mellish Ltd | Southsea | 2020 |
| J P Stone Sales Limited | Holmfirth | 2022 |
| J Palfreeman Rigging Ltd | Middlesbrough | 2022 |
| J RA Rigging Ltd | Middlesbrough | 2026 |
| J Resources Gold (UK) Limited | London | 1984 |
| J T Gillon Distributions Ltd | Salford | 2009 |
| J Walker Industrial Services Ltd | Ardersier | 2025 |
| J Whitfield & SON Decorative Aggregates Limited | Stoke-On-Trent | 2007 |
| J&D Operations Ltd | Canterbury | 2024 |
| J&J Inspection Services Ltd | Durham | 2026 |
| J&M Investments Group Ltd | London | 2025 |
| J&M Ventures Group Ltd | Westhill | 2023 |
| J&S Subsea Limited | Aberdeen | 2019 |
| J-Juniors Ltd | Blackpool | 2024 |
| J. Clubb & SON Limited | Fareham | 1956 |
| J. Suttle Transport Limited | Swanage | 1993 |
| J.& J.Franks Limited | Betchworth | 1941 |
| J.A. Morgan & Sons (Bagillt) Limited | London | 1975 |
| J.A.Jackson Contractors (Leyland) Limited | Blackpool | 2010 |
| J.A.Jackson Contractors(Preston)Limited | Blackpool | 1967 |
| J.C. Mining Ltd. | Airdrie | 2001 |
| J.E.H Mining Construction and General Service Limited | London | 2024 |
| J.M Piping&Mechanical Ltd | Stevenston | 2022 |
| J.Morgan & Sons (Mayobridge) Limited | Belfast | 1966 |
| J.P.E. (Holdings) Limited | Nr Wolverhampton | 1995 |
| J.W.A. Electrical Limited | Elgin | 2012 |
| J2 Subsea Ltd | Glasgow | 2008 |
| J4 Sand & Gravel Ltd | Doncaster | 2024 |
| Jaag Interventions Limited | Montrose | 2022 |
| JAB TEC Consultant Ltd | London | 2026 |
| Jacdaw Engineering Ltd | Nottingham | 2011 |
| Jack UP Consultancy Services Ltd | Montrose | 2024 |
| Jack-Martins Consultancy Ltd. | London | 2021 |
| Jackson Electrical Services Limited | Bridlington | 2013 |
| Jacobs Specialist Water & Civil Engineering Ltd | Coatbridge | 2014 |
| Jacobsen Mechanical Services Ltd | Auchenblae | 2025 |
| Jade Plant and Machinery Ltd | Andover | 2009 |
| Jadestone Energy Holdings Limited | London | 2021 |
| Jadestone Energy PLC | London | 2021 |
| Jadestone Energy U.K. PLC | London | 2025 |
| Jadestone Energy UK Services Ltd | London | 2021 |
| Jaibuildserve Ltd | Epsom | 2025 |
| Jalowiecki Oilfield Engineering Services Ltd | Devizes | 1996 |
| Jamdee Solutions Limited | Dundee | 2018 |
| Jameniola Ltd | Whitley Bay | 2024 |
| James & Sons Distilleries UK Ltd | Ilford | 2020 |
| James Boyd & Sons (Carnmoney) Limited | Newtownabbey | 1934 |
| James Dean Shipping Ltd | London | 2022 |
| James Hendry Slickline Services Ltd | Montrose | 2011 |
| James Jamieson Ardlethen Developments Limited | Ellon | 2009 |
| James Jamieson Quarries Limited | By Ellon | 2010 |
| James Kirk Contracting Ltd | Larbert | 2013 |
| James Mclaren Limited | Glasgow | 2014 |
| James Stevenson (Quarries) Limited | Ballymena | 1991 |
| James Wanstall Ltd | Didcot | 2022 |