| Show Develop Ltd | Information & communication | 2026 |
| SHP Allfresch Limited | Wholesale & retail | 2026 |
| Shree Jalaram Property Ltd | Real estate | 2025 |
| Shri Hari Services Ltd | Wholesale & retail | 2025 |
| Shrove Meadows (Olney) Management Company Limited | Households | 2006 |
| Shub Aram Limited | Other services | 2025 |
| Shuel Construction Limited | Construction | 2018 |
| Shugha Limited | Other services | 2018 |
| Shukrana Transport Limited | Transport & logistics | 2026 |
| Shumena B Events UK Ltd | Extraterritorial | 2020 |
| Shums Tech Limited | Information & communication | 2022 |
| Shung WAA Limited | Real estate | 2026 |
| Shweet Treatz Ltd | Hospitality | 2024 |
| Shyam Services Ltd | Professional & scientific | 2009 |
| SI Buy to LET Ltd | Real estate | 2024 |
| SIA Consultancy Solutions Limited | Professional & scientific | 2010 |
| Siam Rama Thai Massage Ltd | Other services | 2024 |
| Sian&Dia Limited | Transport & logistics | 2026 |
| Sianee BEE Productions Ltd | Professional & scientific | 2020 |
| Siara Togs London Ltd | Wholesale & retail | 2023 |
| Siaree Home Decor Ltd | Wholesale & retail | 2023 |
| SIB Heating & Plumbing Solutions Ltd | Construction | 2026 |
| Sibilia Ltd | Construction | 2026 |
| Sibmat Health Care Resolutions Limited | Public administration | 2020 |
| Sibysoft Ltd | Information & communication | 2015 |
| Sickle & Settle Limited | Wholesale & retail | 2023 |
| Side Ending Ltd | Construction | 2025 |
| Side Hustle Inner Circle Limited | Wholesale & retail | 2026 |
| Side Run Ltd | Transport & logistics | 2024 |
| Sideways Scenery Limited | Construction | 2013 |
| Sidrat Fashion House Limited | Wholesale & retail | 2020 |
| Sidratul Manthahar Ltd | Hospitality | 2020 |
| Sidsha Investments Limited | Real estate | 2013 |
| Sidsol Ltd | Information & communication | 2025 |
| Sienna Beauty Limited | Other services | 2018 |
| Sightline Doors Limited | Manufacturing | 2012 |
| Sightrader Ltd | Information & communication | 2026 |
| Sigma Learning Ltd | Education | 2025 |
| Sigmata Solutions Limited | Information & communication | 2009 |
| Sigmath Ltd | Education | 2022 |
| Signature Accountancy Limited | Professional & scientific | 2024 |
| Signature Aviation Limited | Transport & logistics | 1897 |
| Signature Flight Support London Luton Limited | Transport & logistics | 1988 |
| Signature Reno Limited | Construction | 2022 |
| Signature Security Services Ltd | Administrative & support | 2018 |
| Signatureautos Ltd | Wholesale & retail | 2022 |
| Significant Ventures Ltd | Professional & scientific | 2023 |
| Signoic Ltd | Wholesale & retail | 2025 |
| Signposts (Luton) | Hospitality | 1994 |
| Sigrun Ltd | Wholesale & retail | 2019 |
| Sihube Limited | Health & social care | 2016 |
| Sikhs in Law Assoc. Limited | Other services | 2023 |
| Sikis Ltd | Wholesale & retail | 2025 |
| SIL Logistics Ltd | Construction | 2021 |
| Silas Care Services Ltd | Health & social care | 2020 |
| Silas Services Ltd | Health & social care | 2025 |
| Silicon Gadgets Ltd | Wholesale & retail | 2020 |
| Silk & Snuggle Limited | Wholesale & retail | 2025 |
| Silk Road Properties Ltd | Real estate | 2022 |
| Silk Touch Laser Hair Removal Limited | Other services | 2022 |
| Sillistrations Ltd | Wholesale & retail | 2025 |
| Silsoe Desserts Ltd | Wholesale & retail | 2024 |
| Silsoe Fish Ltd | Wholesale & retail | 2025 |
| Silsoe House Property Services Company Limited | Real estate | 2016 |
| Silver Bells Ltd | Real estate | 2019 |
| Silver Bullet Express Limited | Transport & logistics | 2025 |
| Silver Grove Design & Build Ltd | Construction | 2017 |
| Silver Mane Ltd | Professional & scientific | 2024 |
| Silver Star Security Ltd | Construction | 2019 |
| Silver Starz Estates Ltd | Real estate | 2025 |
| Silver Stone Recruitment Ltd | Administrative & support | 2025 |
| Silver Streak Recruitment and Learning Agency CIC | Education | 2010 |
| Silver Streak Recruitment and Learning Ltd. | Education | 2012 |
| Silverback Productions Limited | Extraterritorial | 2021 |
| Silvercrown Luton Limited | Real estate | 2008 |
| Silverhill Estates Limited | Real estate | 2004 |
| Silverline Private Car Hire Ltd | Administrative & support | 2018 |
| Silvermist Investments Limited | Real estate | 2015 |
| Silversteam Ltd | Wholesale & retail | 2024 |
| Silverstone Solicitors Limited | Professional & scientific | 2017 |
| Silvertoad Ltd | Information & communication | 2008 |
| Silviuruben Limited | Transport & logistics | 2024 |
| Silvr-Line Limited | Wholesale & retail | 2025 |
| Simaya PVT Limited | Health & social care | 2022 |
| Simbah Sports Limited | Wholesale & retail | 2019 |
| Simi Craft Limited | Arts & recreation | 2025 |
| Simion Building Services Limited | Construction | 2014 |
| Simion P Construct Ltd | Construction | 2017 |
| Simmic Electrical Ltd | Construction | 2025 |
| Simon Adrian Contractors Ltd | Construction | 2012 |
| Simon Cooper & Co Limited | Professional & scientific | 2023 |
| Simon Estate Ltd | Real estate | 2024 |
| Simon Real Estates Limited | Construction | 2017 |
| Simon Tarbox Limited | Financial services | 2022 |
| Simon-Hartman London Ltd | Wholesale & retail | 2016 |
| Simple Carz Ltd | Administrative & support | 2021 |
| Simple DOT Digital Ltd | Information & communication | 2019 |
| Simple Hire Cars Ltd | Administrative & support | 2018 |
| Simple Made Services Ltd | Wholesale & retail | 2024 |
| Simple Media Solutions Trading Company Ltd | Manufacturing | 2025 |