| SI Holdings & Investments Ltd | Real estate | 2020 |
| SI Realisations 2022 Limited | Manufacturing | 1965 |
| SI Smart Innovation Ltd | Professional & scientific | 2024 |
| SIA Coffee Ltd | Wholesale & retail | 2026 |
| SIA Demerged Ltd | Financial services | 2025 |
| SIA Enterprises Limited | Other services | 2006 |
| Sian Barton Comms Ltd | Information & communication | 2025 |
| Sianmillah Enterprise Limited | Health & social care | 2015 |
| Siberian Chic Limited | Wholesale & retail | 2013 |
| Sibley and Ward Ltd | Administrative & support | 2021 |
| Sibtains East Ltd | Hospitality | 2022 |
| Sibtains Ltd | Manufacturing | 2016 |
| Sicura Homes Ltd | Financial services | 2023 |
| SID & REY Investments Ltd | Real estate | 2021 |
| SID Telfers Limited | Wholesale & retail | 2023 |
| Sid's Autos Ltd | Administrative & support | 2014 |
| Sid's Cars Limited | Wholesale & retail | 2015 |
| Siddhanth Limited | Information & communication | 2018 |
| Side Hustle Ltd | Professional & scientific | 2021 |
| Side Hustle Pictures Ltd | Extraterritorial | 2023 |
| Side Shoots Limited | Information & communication | 2014 |
| Sidewalk Hospitality Ltd | Hospitality | 2026 |
| Sidh Homes Ltd | Real estate | 2026 |
| Sidmancer Limited | Administrative & support | 2024 |
| Sidney Trading Limited | Wholesale & retail | 2023 |
| Sidovations Limited | Professional & scientific | 2015 |
| Sidsan Holdings Limited | Real estate | 2019 |
| Sidwil Services Limited | Wholesale & retail | 2013 |
| Siebke Design Limited | Professional & scientific | 2026 |
| Siempre Juntos Limited | Administrative & support | 2019 |
| Sievemk Gateway Ltd | Education | 2011 |
| Sifar Accounting Ltd | Professional & scientific | 2022 |
| Sigh Holdings Ltd | Professional & scientific | 2026 |
| Sigh Technologies Limited | Financial services | 2018 |
| Sigma Natby Limited | Wholesale & retail | 2023 |
| Sigmahq Ltd | Information & communication | 2014 |
| Sign T Ltd | Transport & logistics | 2013 |
| Signature Developments MK Ltd | Construction | 2019 |
| Signature Dolores Homes Ltd | Construction | 2025 |
| Signature Faces Limited | Other services | 2021 |
| Signature Fundraising Ltd | Wholesale & retail | 2013 |
| Signature Home Interiors MK Ltd | Construction | 2025 |
| Signature Homes MK LLP | | 2014 |
| Signature Installations Ltd | Construction | 2025 |
| Signature Land (Stoke) Ltd | Construction | 2022 |
| Signature Land Ltd | Construction | 2017 |
| Signature Studios Aesthetics Ltd | Other services | 2021 |
| Significant Scenes Ltd | Information & communication | 2024 |
| Sigstone Limited | Professional & scientific | 2016 |
| Sigull Ltd | Wholesale & retail | 2020 |
| Sihhee Ltd | Wholesale & retail | 2024 |
| Siju Limited | Construction | 2005 |
| SIK Maintenance Ltd | Manufacturing | 2025 |
| SIK Transport Ltd | Transport & logistics | 2015 |
| Sikabaa Limited | Wholesale & retail | 2025 |
| Sike Futures Ltd | Health & social care | 2025 |
| SIL Realisations 2013 Limited | Manufacturing | 1973 |
| Silbury 272 Limited | Hospitality | 2003 |
| Silbury Estates & Property Managment Ltd | Households | 2025 |
| Silent Monkey Computer Services Ltd | Manufacturing | 1998 |
| Silentnoir Ltd | Manufacturing | 2026 |
| Silk Pipes Ltd | Hospitality | 2024 |
| Silks Solicitors Limited | Professional & scientific | 2011 |
| Silmaril Ltd | Professional & scientific | 2020 |
| Siloc Creativ Ltd. | Administrative & support | 2016 |
| Silva Estates Ltd | Real estate | 2024 |
| Silver Beach Creations Limited | Manufacturing | 2019 |
| Silver FOX Vehicles Limited | Wholesale & retail | 2025 |
| Silver Mountain Coffee Ltd | Manufacturing | 2018 |
| Silver Oak Strategic Innovations Ltd | Professional & scientific | 2025 |
| Silver Online Ltd | Wholesale & retail | 2019 |
| Silver Prestige Limited | Information & communication | 2022 |
| Silver Street Automotive Limited | Wholesale & retail | 1962 |
| Silverbury Limited | Real estate | 1983 |
| Silverdale Consulting Ltd | Wholesale & retail | 2000 |
| Silverdove Services Ltd | Health & social care | 2015 |
| Silverlake Investments London Ltd | Construction | 2022 |
| Silverline Cleaning Services Limited | Administrative & support | 2025 |
| Silverline Electrical Ltd | Manufacturing | 2024 |
| Silverline GAS & Renewables Ltd | Construction | 2023 |
| Silverline Holding Group Ltd | Households | 2024 |
| Silverline Plumbing GAS & Electrical Limited | Construction | 2019 |
| Silvermanns Ltd | Administrative & support | 2011 |
| Silvernet Limited | Manufacturing | 2005 |
| Silverspade Landscapes Limited | Administrative & support | 2019 |
| Silverspade Property Limited | Real estate | 2024 |
| Silverstar UK Trade Limited | Transport & logistics | 2008 |
| Silverstone International Ltd | Real estate | 2015 |
| Silverstone RS21 Limited | Professional & scientific | 2020 |
| Silverstone Signs Limited | Financial services | 2023 |
| Silvertouch Craftsmanship Ltd | Construction | 2017 |
| Silvertown Tunnel Holdings Limited | Financial services | 2019 |
| Silvertrax Ltd | Transport & logistics | 2024 |
| Silverwax Holdings Ltd | Wholesale & retail | 2025 |
| SIM 3D Racing Ltd | Manufacturing | 2022 |
| Simba Medical Consultancy Limited | Health & social care | 2024 |
| Simbah Academy Limited | Education | 2025 |
| Simbah Clothing Ltd | Manufacturing | 2023 |
| Simetrics Ltd | Information & communication | 2002 |
| Simibo Ltd | Professional & scientific | 2021 |