| Sighiartau Limited | Transport & logistics | 2015 |
| Sightbywings Limited | Health & social care | 2011 |
| Sign&Maintenance Ltd | Construction | 2026 |
| Signal Taxi Company Ltd | Transport & logistics | 2026 |
| Signalling & IT Solutions Ltd | Professional & scientific | 2023 |
| Signature Metal Artwork Limited | Wholesale & retail | 2025 |
| Signature Roofing Limited | Construction | 2008 |
| Signature Wealth Ltd | Financial services | 2017 |
| Sigwick Media Ltd | Information & communication | 2022 |
| Sikhingit Ltd | Professional & scientific | 2011 |
| Silen Care Services Ltd | Health & social care | 2022 |
| Silent Arena Ltd | Extraterritorial | 2022 |
| Silent Disco King Limited | Extraterritorial | 2012 |
| Silent Productions Ltd | Information & communication | 2025 |
| Silent Rooms Ltd | Information & communication | 2008 |
| Silentflow Ltd | Professional & scientific | 2025 |
| Siley & Smith Diesel Services Limited | Professional & scientific | 1977 |
| Silk Pine UK Ltd | Wholesale & retail | 2024 |
| Silk Road Mountain Race Limited | Administrative & support | 2015 |
| Silkbet Limited | Arts & recreation | 2025 |
| Silkcut Limited | Professional & scientific | 2018 |
| Silkfred Limited | Other services | 2011 |
| Silkstart Limited | Manufacturing | 2025 |
| Silly LEG Bakery Ltd | Manufacturing | 2023 |
| Silver Arrow MB Limited | Wholesale & retail | 2010 |
| Silver Assets Management Ukeig | | 2012 |
| Silver Celebrations Limited | Wholesale & retail | 2012 |
| Silver Fern Limited | Professional & scientific | 1984 |
| Silver Lining Matters Ltd. | Other services | 2013 |
| Silver Maple Ltd | Extraterritorial | 2017 |
| Silver Pavillon Ukeig | | 2008 |
| Silver Sail Yacht Surveyors Ltd | Professional & scientific | 2010 |
| Silver Sea Seafood Ltd | Wholesale & retail | 2021 |
| Silver Service Apartments Ltd | Transport & logistics | 2021 |
| Silver Shield Associates Limited | Professional & scientific | 2017 |
| Silverbloom Technology Limited | Information & communication | 2025 |
| Silveroak Solutions Ltd | Information & communication | 2025 |
| Silverock Enterprises Limited | Wholesale & retail | 2003 |
| Silversprings Garden Centre Limited | Wholesale & retail | 2009 |
| Silversun Studios Ltd | Information & communication | 2025 |
| Silverwing Renovations Ltd | Construction | 2025 |
| Silverwoods Construction Ltd | Construction | 2011 |
| Silverwoods South Limited | Construction | 2020 |
| Silvery Jewellery Limited | Manufacturing | 2022 |
| Silvretta Court Management Limited | Households | 1977 |
| Simba Cleans Ltd | Administrative & support | 2024 |
| Simba Coaching Ltd | Professional & scientific | 2021 |
| Simbashopping UK Limited | Wholesale & retail | 2022 |
| Simcal Limited | Extraterritorial | 2018 |
| Simeu Sports Limited | Professional & scientific | 2020 |
| Simm Subsea Limited | Professional & scientific | 2018 |
| Simmojit Limited | Information & communication | 2023 |
| Simmonds (International) Financial Associates Limited | Administrative & support | 1996 |
| Simms Insights Consulting Ltd | Information & communication | 2026 |
| Simon Baker Property Limited | Construction | 2019 |
| Simon Fernandes Property Consultants Limited | Real estate | 2016 |
| Simon Griffiths Plumbing and Heating Limited | Construction | 2025 |
| Simon John Photographs Limited | Professional & scientific | 2003 |
| Simon K. Lewis Photography Ltd | Professional & scientific | 2025 |
| Simon Services Ltd | Manufacturing | 2026 |
| Simon Udal Design Limited | Professional & scientific | 2003 |
| Simon Watkins Consulting Limited | Professional & scientific | 2004 |
| Simon's Executive Journeys Ltd | Transport & logistics | 2022 |
| Simple Asbestos Solutions Limited | Water & waste | 2018 |
| Simple Cruise Parking Ltd | Transport & logistics | 2025 |
| Simple Design Services Ltd | Extraterritorial | 2018 |
| Simple Ltd | Education | 2020 |
| Simple Support Solutions Ltd | Information & communication | 2023 |
| Simplefit Limited | Manufacturing | 1978 |
| Simplicoat Ltd | Manufacturing | 2022 |
| Simply Cured Ltd | Hospitality | 2024 |
| Simply FIX Ltd | Other services | 2013 |
| Simply Made Company Ltd | Manufacturing | 2014 |
| Simply Office 365 Ltd | Information & communication | 2018 |
| Simply Speaking UK Limited | Health & social care | 2009 |
| Simply Stocking Limited | Wholesale & retail | 2019 |
| Simplyready Ltd | Information & communication | 2026 |
| Simpson Capital Group Ltd | Financial services | 2025 |
| Simpson Logistic Services Limited | Transport & logistics | 2013 |
| Simpson Property Investment Group Ltd | Real estate | 2025 |
| Simtax Accounting Limited | Extraterritorial | 2017 |
| SIN Free Foods Ltd | Transport & logistics | 2017 |
| Sinag Pinoy Ltd | Construction | 2025 |
| Sinclair Simpson Ltd | Wholesale & retail | 2016 |
| Sindria Ltd | Transport & logistics | 2019 |
| Sinfor International Ltd | Wholesale & retail | 2026 |
| Singh & Singh Investment Limited | Real estate | 2025 |
| Singh Engineer Limited | Professional & scientific | 2019 |
| Singhkaur Holdings Limited | Wholesale & retail | 2006 |
| Sintim Limited | Health & social care | 2016 |
| Siobhan O’Reilly People Solutions Ltd | Professional & scientific | 2023 |
| Siobhan Properties Limited | Real estate | 2026 |
| Siore Ltd | Wholesale & retail | 2025 |
| SIP & Save Ltd | Wholesale & retail | 2025 |
| Siquta Ltd | Administrative & support | 2025 |
| Siratx Limited | Wholesale & retail | 2025 |
| Siratx Motors Ltd | Wholesale & retail | 2025 |
| Siren Medical Services Ltd | Health & social care | 2023 |
| Sirius About Pets Ltd | Wholesale & retail | 2025 |
| Sirius Research Ltd. | Professional & scientific | 2000 |