| Sibr Enterprises Ltd | Real estate | 2024 |
| Sibusiso Medicare Ltd | Health & social care | 2013 |
| Sichi Ltd | Health & social care | 2023 |
| Side a Advisory Ltd | Professional & scientific | 2026 |
| Siem&S Ltd | Transport & logistics | 2018 |
| Sienna Capital Limited | Professional & scientific | 2008 |
| Sienna Estate Management Ltd | Real estate | 2023 |
| Sienna Labs Ltd | Manufacturing | 2025 |
| Sifat Ventures Ltd | Professional & scientific | 2026 |
| Sig-Nexus Limited | Information & communication | 2025 |
| Sigered Limited | Construction | 2006 |
| Sigma Contracts Limited | Professional & scientific | 2015 |
| Sigma Electrical Services (Essex) Ltd | Construction | 2006 |
| Sigma Tree Ltd | Hospitality | 2021 |
| Sigma Tree Technologies Ltd | Professional & scientific | 2024 |
| Sigmaone Limited | Administrative & support | 2005 |
| Sign-It (UK) Limited | Manufacturing | 2011 |
| Signature J & C Ltd | Manufacturing | 2026 |
| Signature Smile TC Ltd | Health & social care | 2025 |
| Signbrix Ltd | Manufacturing | 2026 |
| Signtec Limited | Administrative & support | 2001 |
| SII Bello Collective Ltd | Other services | 2026 |
| Siireo Limited | Extraterritorial | 2024 |
| Sikho Solutions Limited | Health & social care | 2017 |
| Silent Lake Studios Ltd | Information & communication | 2021 |
| Silent Notes Ltd | Wholesale & retail | 2025 |
| Silhouette DU Barry Limited | Other services | 1983 |
| Silk Helix Ltd | Administrative & support | 2019 |
| Silk Render Limited | Construction | 2017 |
| Silowe Properties Limited | Real estate | 2025 |
| Silver Fern Health Ltd | Health & social care | 2016 |
| Silver Food Ltd | Wholesale & retail | 2014 |
| Silver Lace Boutique Ltd | Wholesale & retail | 2017 |
| Silver Reach Media Limited | Professional & scientific | 2024 |
| Silver Shield Trading Ltd | Wholesale & retail | 2012 |
| Silver Touch Technologies (UK) Limited | Information & communication | 2007 |
| Silvergate Europe Limited | Information & communication | 2010 |
| Silversky Contracts Ltd | Agriculture & fishing | 2015 |
| Silvest Asset Management Limited | Professional & scientific | 2025 |
| SIM Healthcare Limited | Health & social care | 2018 |
| Simmtex Group Services Ltd | Financial services | 2025 |
| Simon Petherick Graphic and Web Design Ltd | Professional & scientific | 2012 |
| Simon Rudge Motorsport Ltd | Wholesale & retail | 2015 |
| Simpatico Audio Post Limited | Information & communication | 2014 |
| Simple Hive Recruitment Ltd | Administrative & support | 2025 |
| Simple Signman Property Limited | Real estate | 1992 |
| Simple Utilities Ltd | Professional & scientific | 2024 |
| Simple Way to Go Ltd | Other services | 2020 |
| Simpletext Ltd | Information & communication | 2026 |
| Simply Acorn Group Limited | Administrative & support | 2016 |
| Simply Boreham Limited | Wholesale & retail | 2021 |
| Simply Chapters Limited | Hospitality | 2017 |
| Simply Fitted Furniture Limited | Construction | 2017 |
| Simply Lending Solutions Limited | Financial services | 2015 |
| Simply Logic Limited | Real estate | 1998 |
| Simply Promotion Ltd | Professional & scientific | 2021 |
| Simtek (2012) Ltd | Manufacturing | 2012 |
| Simulated Learning Solutions Ltd | Education | 2014 |
| Simvas Ltd | Transport & logistics | 2014 |
| Sinclair Leisure Property Holdings Limited | Financial services | 2023 |
| Sinclair Property Investments Ltd | Real estate | 2017 |
| Sinclair Skin Services Ltd | Health & social care | 2018 |
| Sinead Nicholson Education Consultancy Ltd | Education | 2024 |
| Sinecosse UK Ltd | Professional & scientific | 2022 |
| Sineqn Capital Limited | Real estate | 2015 |
| Sineqn Ltd | Professional & scientific | 2003 |
| Singh DS Holdings Ltd | Wholesale & retail | 2024 |
| Sinyoro Limited | Professional & scientific | 2023 |
| SIR Evelyn Wood Limited | Hospitality | 2023 |
| SIR ROY Stone Limited | Professional & scientific | 2024 |
| Sirac Barbers Limited | Other services | 2019 |
| Sirgan Consultants Limited | Professional & scientific | 2005 |
| Sirgan Property Investments Limited | Real estate | 2007 |
| Sirm UK Marine Limited | Information & communication | 2016 |
| Sisken Limited | Administrative & support | 2025 |
| Sisra Limited | Administrative & support | 2008 |
| Sisu Retail Group Limited | Wholesale & retail | 2023 |
| Sitech Developments Ltd | Construction | 2019 |
| SIX Hills Management Limited | Real estate | 2015 |
| SIX Mile Bottom Entertainment Ltd | Hospitality | 2022 |
| SIX Mile Bottom Pursuits Limited | Financial services | 2022 |
| SIX Sons Limited | Construction | 2013 |
| Sixteen Solutions Ltd | Professional & scientific | 2026 |
| SJ Apparel Sales Ltd | Wholesale & retail | 2022 |
| SJ Event Caterers Limited | Wholesale & retail | 2019 |
| SJ O'Connor Cladding Limited | Construction | 2018 |
| SJ Prime Properties Ltd | Real estate | 2024 |
| SJ Snap Limited | Transport & logistics | 2025 |
| SJ3 Property Limited | Real estate | 2026 |
| SJA Logistics Ltd | Transport & logistics | 2017 |
| SJB Book-Keeping & Accountancy Services Ltd | Professional & scientific | 2025 |
| SJB Floodlighting Limited | Other services | 2008 |
| SJC Asset Management Limited | Administrative & support | 2019 |
| SJC Roofing and Maintenance Limited | Administrative & support | 2017 |
| SJH Utilities Limited | Construction | 2015 |
| SJM Electro-Solutions Limited | Manufacturing | 2010 |
| Sjogati Limited | Wholesale & retail | 2021 |
| SJR Transport Services Ltd | Transport & logistics | 2024 |
| Sjrlmr Ltd | Information & communication | 2022 |
| SJS Power Engineering Ltd | Professional & scientific | 2018 |