| Hertfordshire Multiple Sclerosis Therapy Centre Ltd. | Health & social care | 1988 |
| Hertfordshire Refurbishment Ltd | Construction | 2017 |
| Herts & Beds Electronic Services Limited | Wholesale & retail | 2010 |
| Herts at Home Limited | Health & social care | 2018 |
| Herts Precision Surface Repairs Limited | Other services | 2026 |
| Herts Stairlifts Ltd | Professional & scientific | 2023 |
| Hertsgate Signs Ltd | Administrative & support | 2025 |
| HFR Marine Ltd | Other services | 2014 |
| HGX Systems Ltd | Information & communication | 2013 |
| Hicce Limited | Real estate | 2023 |
| Hickling Developments Limited | Wholesale & retail | 2020 |
| High Performance Technology UK Limited | Professional & scientific | 2018 |
| High Point Hitchin (Maintenance) Limited | Households | 1968 |
| High Voltage Commissioning Ltd | Construction | 2025 |
| Highlands Village Pre-School Limited | Education | 2005 |
| HIL Developments Ltd | Construction | 2021 |
| Hillbrow 1969 Limited | Extraterritorial | 2015 |
| Hilton House Properties Limited | Construction | 1973 |
| Hiraeth Estates Limited | Real estate | 2025 |
| Hiretowire Ltd | Construction | 2025 |
| Hitchin and Letchworth Performing Arts Ltd | Arts & recreation | 2026 |
| Hitchin Bookkeeper Limited | Information & communication | 2010 |
| Hitchin Investments Limited | Real estate | 2024 |
| Hitchin Road (Stotfold) Management Limited | Real estate | 2018 |
| HLW Consultants Ltd | Professional & scientific | 2026 |
| HM & Daughters Property Investments Limited | Real estate | 2021 |
| Holland Motor Company Ltd | Wholesale & retail | 2015 |
| Holt Music Limited | Information & communication | 2021 |
| Home@Heart Limited | Real estate | 2025 |
| Homestech Ltd | Real estate | 2024 |
| Hope & Co Holdings Ltd | Construction | 2025 |
| Hope Esperanza Limited | Real estate | 2008 |
| Hope Esperanza Properties Ltd | Real estate | 1997 |
| Hopkins Financial Services Limited | Financial services | 2016 |
| Hopscotch Property Ltd | Real estate | 2024 |
| Hopton Capital Limited | Real estate | 2019 |
| Howard Read Writes Ltd | Arts & recreation | 2024 |
| Howcrofts Consulting Ltd | Professional & scientific | 2018 |
| Howes and Boughton (Letchworth) Limited | Construction | 2007 |
| Hsdw Ltd | Construction | 2024 |
| HSF Rentals Ltd | Hospitality | 2025 |
| HSM Aero Limited | Manufacturing | 2003 |
| Hssc Limited | Administrative & support | 2013 |
| HT Locksmiths Limited | Administrative & support | 2025 |
| Httk Property Limited | Professional & scientific | 2017 |
| Hudson Huckle Construction Ltd | Construction | 2022 |
| Hunter Bond Limited | Administrative & support | 2014 |
| Hutch Marketing Ltd | Professional & scientific | 2021 |
| Hutkese Ltd | Hospitality | 2026 |
| HWW Group Limited | Financial services | 2017 |
| HWW Investments Ltd | Real estate | 2022 |
| Hydrolow Limited | Construction | 2023 |
| Hydropro UK Ltd | Construction | 2025 |
| Hype Clothing Co Ltd | Wholesale & retail | 2025 |
| Hype Health Hub Ltd | Arts & recreation | 2023 |
| Hyperdyne Systems Limited | Other services | 2015 |
| HZ Residential Limited | Real estate | 2026 |
| I V Garage Services Ltd | Manufacturing | 2016 |
| I-75 Limited | Professional & scientific | 2009 |
| I-Gear Devices Limited | Information & communication | 2013 |
| Icdm Consulting Ltd | Professional & scientific | 2020 |
| Icon Connect Limited | Construction | 1987 |
| Icor Limited | Wholesale & retail | 2004 |
| Iffr (UK) Limited | Other services | 2003 |
| Ifix Computing Ltd | Wholesale & retail | 2012 |
| Ignite Local Ltd | Education | 2023 |
| Illume.Plus Limited | Professional & scientific | 2010 |
| Imagine the Future Consulting Limited | Administrative & support | 2007 |
| Imagine the Future Holdings Limited | Financial services | 2026 |
| Imajica Theatre Company CIC | Education | 2015 |
| Imperial Moves Limited | Transport & logistics | 2015 |
| Imri London Ltd | Education | 2021 |
| Imri Ltd | Education | 2021 |
| In-Home Carers Limited | Other services | 2012 |
| INB Logistics Ltd | Transport & logistics | 2017 |
| Independent Law Brokers Limited | Administrative & support | 1993 |
| Inferno Communications Ltd | Information & communication | 2018 |
| Influenca Limited | Information & communication | 2001 |
| Inkviro Limited | Water & waste | 2007 |
| Inlineav Ltd | Administrative & support | 2016 |
| Innovative Solutions Group Limited | Financial services | 2022 |
| Innovative Trials Limited | Professional & scientific | 2010 |
| Innovex Investments Limited | Real estate | 2024 |
| Innventure Limited | Hospitality | 2003 |
| Inscape Aviation Ltd | Transport & logistics | 2012 |
| Insight Visual Systems Limited | Other services | 1991 |
| Insite Associates Limited | Information & communication | 2014 |
| Inspired Hill Top Limited | Wholesale & retail | 2021 |
| Integra Signs Ltd | Administrative & support | 2014 |
| Integrate Properties Ltd | Real estate | 2026 |
| Integrate Social Ltd | Professional & scientific | 2024 |
| Intellordata Ltd | Information & communication | 2022 |
| Intercede Associates UK Limited | Professional & scientific | 2013 |
| Intercede Security Limited | Administrative & support | 2020 |
| International Audiophile Recording Ltd | Information & communication | 2008 |
| International Tooling Limited | Wholesale & retail | 2005 |
| Intertest Management Limited | Information & communication | 2011 |
| Into AV Distribution Limited | Wholesale & retail | 2015 |
| Into AV Limited | Wholesale & retail | 2003 |
| Invista Electrical Services Ltd | Construction | 2020 |