| Siem Capital UK Limited | Financial services | 1904 |
| Siem OIL Service Invest Holdings Limited | Administrative & support | 2016 |
| Siem OIL Service Invest Limited | Administrative & support | 2016 |
| Siem Shipowning Roro Limited | Transport & logistics | 1991 |
| Siem Shipping UK Limited | Transport & logistics | 1993 |
| Siena Grace Ltd | Extraterritorial | 2020 |
| Sienna Services Limited | Health & social care | 2019 |
| Sierra Leone Vulnerable and Physically Challenged CIC | Other services | 2021 |
| Sign & Media UK Limited | Manufacturing | 2013 |
| Sign Grafx Display Limited | Other services | 2021 |
| Signal Finance Limited | Professional & scientific | 2016 |
| Signature Accountants Ltd | Professional & scientific | 2018 |
| Signature Villages Limited | Real estate | 2024 |
| Siie Property Management Ltd | Real estate | 2022 |
| Sikandar Limited | Wholesale & retail | 2025 |
| Silent Wick Ltd | Wholesale & retail | 2026 |
| Siliyan Ltd | Wholesale & retail | 2026 |
| Silk E-Way Ltd | Wholesale & retail | 2020 |
| Silk Road Manufacturing UK Limited | Extraterritorial | 2010 |
| Silva & Faber Ltd | Extraterritorial | 2015 |
| Silva Construction & Interiors Ltd | Extraterritorial | 2015 |
| Silvamore Limited | Construction | 2010 |
| Silver Anchor Properties Limited | Real estate | 2023 |
| Silver Crescent Development Ltd | Real estate | 2025 |
| Silver Heating Limited | Construction | 2019 |
| Silver Screen SET Co Ltd | Information & communication | 2025 |
| Silver Sparkle Productions Limited | Arts & recreation | 2016 |
| Silverback Estates Ltd | Real estate | 2021 |
| Silverback Lighting Limited | Energy | 2025 |
| Silverback Publishing Ltd | Information & communication | 2013 |
| Silverback Roofing Ltd. | Construction | 2020 |
| Silverback Scaffolding Contractors Ltd | Construction | 2025 |
| Silverback Waste and Recycling Company Ltd | Water & waste | 2023 |
| Silvercroft Consultancy Limited | Professional & scientific | 2020 |
| Silverwood Property Services Ltd | Professional & scientific | 2017 |
| SIM (PVT) Limited | Transport & logistics | 2023 |
| Simaim Limited | Real estate | 2021 |
| Simbolt Holdings Ltd | Financial services | 2023 |
| Simbolt Investments Ltd | Administrative & support | 2023 |
| Simon Oram Ltd | Transport & logistics | 2015 |
| Simple Site Solutions Limited | Administrative & support | 2024 |
| Simple World Shop Ltd | Wholesale & retail | 2023 |
| Simply Business Products Ltd | Manufacturing | 2013 |
| Simply Cremation and Burial Limited | Other services | 2022 |
| Simply Prizes Limited | Professional & scientific | 2005 |
| Simply Sash Windows and Doors Limited | Construction | 2020 |
| Simply Tudor Tours Limited | Education | 2023 |
| Simpson Engineering Limited | Manufacturing | 2022 |
| Sinfonia Diverse Ltd | Education | 2025 |
| Sinfonic Oxshott Ltd | Real estate | 2025 |
| Singapore Ltd | Health & social care | 2021 |
| Singh Trader Limited | Wholesale & retail | 2023 |
| Sinnertons Limited | Real estate | 2013 |
| Sino Communication Ltd | Professional & scientific | 2019 |
| Sipd Properties Limited | Real estate | 2021 |
| Sirmon Industries Limited | Manufacturing | 2011 |
| Sirona International Limited | Administrative & support | 2018 |
| Sirton Computer Services Limited | Information & communication | 1998 |
| SIS Staffing Limited | Administrative & support | 2025 |
| Sister ACT International TOO Ltd | Arts & recreation | 2025 |
| Sister ACT Theatre Limited | Arts & recreation | 2013 |
| Sister ACT West END Ltd | Arts & recreation | 2023 |
| Site Manage Limited | Construction | 2011 |
| Site Supply Direct Limited | Administrative & support | 2016 |
| Sithsan Illam Ltd | Real estate | 2025 |
| Siver London Ltd | Hospitality | 2023 |
| SIX Planets Ltd | Construction | 2024 |
| Sixi Ltd | Hospitality | 2022 |
| SJB Protection Ltd | Administrative & support | 2025 |
| SJC Consulting London Limited | Administrative & support | 2015 |
| SJC Motors Ltd | Wholesale & retail | 2023 |
| SJC Nutrition Ltd | Administrative & support | 2018 |
| Sjee Residence Ltd | Real estate | 2020 |
| SJH Consulting and Design Services Limited | Professional & scientific | 2023 |
| SJK Property Holding Ltd | Real estate | 2022 |
| Sjpp Ltd | Transport & logistics | 2018 |
| SK Building Services (Sutton) Ltd | Construction | 2026 |
| SK Cash and Carry Ltd | Wholesale & retail | 2025 |
| Skadrock Limited | Construction | 2019 |
| Skanda Global Private Limited | Wholesale & retail | 2024 |
| Skansys Consultancy Ltd | Information & communication | 2014 |
| Skein Advisory UK Ltd | Professional & scientific | 2021 |
| Skein Global Ltd | Financial services | 2021 |
| Skelton Business Services Limited | Administrative & support | 2018 |
| Skillman Limited | Wholesale & retail | 2018 |
| Skillsbridge Innovations Ltd | Information & communication | 2025 |
| Skin Wellbeing Ltd | Other services | 2025 |
| Skinfidence Ltd | Manufacturing | 2022 |
| Skirtify Ltd | Construction | 2025 |
| Sknly.Ldn Limited | Other services | 2026 |
| SKP Ventures Ltd | Professional & scientific | 2026 |
| SKS Consultants Ltd | Administrative & support | 2022 |
| Sky Roofing Specialists Ltd | Construction | 2026 |
| Skybuild (Surrey) Ltd | Construction | 2017 |
| Skycastle Limited | Extraterritorial | 2012 |
| Skylight Factory Ltd | Wholesale & retail | 2024 |
| Skylight Luminous Limited | Construction | 2026 |
| Skylink Group Limited | Transport & logistics | 2013 |
| Skylit Ltd | Wholesale & retail | 2021 |
| Skystar Cricket Club Ltd | Education | 2024 |