| Sid.Graphics Limited | Information & communication | 2000 |
| Sifly Ltd | Arts & recreation | 2016 |
| Signage Plus UK Ltd | Professional & scientific | 2019 |
| Signature Renovations London Ltd | Construction | 2022 |
| Signs of the Times (GB) Limited | Other services | 2009 |
| Silk Hair Group Ltd | Other services | 2024 |
| Silko Trading Ltd | Wholesale & retail | 2025 |
| Silver-Tongued Charmers Ltd | Information & communication | 2022 |
| Silverdale Professional Services Ltd | Information & communication | 2016 |
| Silverforce Ltd | Wholesale & retail | 1996 |
| Silvertime Limited | Administrative & support | 2009 |
| Silvertown SC Ltd | Professional & scientific | 2020 |
| Silverware Technology Limited | Information & communication | 2024 |
| Silverwood Capital Limited | Financial services | 2010 |
| Silvestris Legal Consultancy Ltd | Professional & scientific | 2022 |
| Simi Decorating Ltd | Construction | 2023 |
| Simon Delaney Associates Ltd | Professional & scientific | 2019 |
| Simon Hughes Sports Limited | Arts & recreation | 2019 |
| Simon Nikolli Properties Ltd | Real estate | 2025 |
| Simone Close Residents Limited | Extraterritorial | 1984 |
| SIMP4TEA Ltd | Wholesale & retail | 2022 |
| Simpkins & Pook Ltd | Arts & recreation | 2023 |
| Simplicity Group London Limited | Administrative & support | 2023 |
| Simplicityfirst Limited | Other services | 2019 |
| Simply Drapes Limited | Other services | 2001 |
| Simply Plumbing Bathrooms Limited | Wholesale & retail | 2016 |
| Simply Plumbing Group Ltd | Construction | 2023 |
| Simply Soft Play Limited | Arts & recreation | 2026 |
| Simply Underfloor Heating Systems Limited | Wholesale & retail | 2025 |
| Sinclair London Ltd | Extraterritorial | 2025 |
| Sinclair Surfaces Ltd | Construction | 2021 |
| Sindhu Investments Ltd | Real estate | 2014 |
| Sinead Armstrong Consulting Limited | Professional & scientific | 2019 |
| Sing a Book Community Interest Company | Education | 2014 |
| Singletons Technology Ltd | Information & communication | 2024 |
| Sipeople Ltd | Information & communication | 2025 |
| Sipwell Coffee Limited | Wholesale & retail | 2023 |
| Sired Ltd | Financial services | 2013 |
| Sirolo Limited | Arts & recreation | 2013 |
| Siteview Construction Limited | Construction | 2024 |
| Sitnas Ltd | Construction | 2017 |
| Siva Shop Ltd | Wholesale & retail | 2023 |
| Sivien Limited | Wholesale & retail | 2022 |
| Sivo Limited | Hospitality | 2023 |
| Sivy Support Services Limited | Information & communication | 2025 |
| Siza Productions Limited | Manufacturing | 2025 |
| SJ Fitch & Co Ltd | Real estate | 2024 |
| SJB Ventures (UK) Limited | Real estate | 2019 |
| SJH Accountants Holdings Limited | Financial services | 2025 |
| SJH Accountants Limited | Professional & scientific | 2018 |
| SK Brickwork Ltd | Construction | 2018 |
| Skelton Lodge RTM Company Limited | Households | 2024 |
| Skeme Limited | Construction | 2014 |
| Skerryvore Technology Limited | Administrative & support | 2010 |
| Skill ME NOW Ltd | Education | 2024 |
| Skillcrown Holdings (2018) Limited | Financial services | 2018 |
| Skillcrown Holdings Limited | Construction | 2013 |
| Skillcrown Homes (UK) LLP | | 2009 |
| Skillcrown Homes Limited | Construction | 1993 |
| Skillcrown Ltd | Construction | 2018 |
| Skills and Strategies Ltd | Other services | 2023 |
| Skilzmansion Ltd | Administrative & support | 2022 |
| Skin Dr-Ime Ltd | Wholesale & retail | 2024 |
| Skinny's Plastering Limited | Construction | 2021 |
| SKM Properties Limited | Hospitality | 2011 |
| Skogan Ventures Limited | Professional & scientific | 2023 |
| Skopti Critical Minerals Ltd | Mining & quarrying | 2025 |
| Skout Incorporated Limited | Information & communication | 2025 |
| SKP88 Ltd | Construction | 2015 |
| Skuggan Ltd | Professional & scientific | 2024 |
| Sky Express Ltd | Construction | 2018 |
| Sky House Construction Limited | Construction | 2020 |
| Sky Jenesis Entertainment Ltd | Administrative & support | 2019 |
| Sky Jenesis Ltd | Information & communication | 2019 |
| Sky Jenesis Transport Ltd | Mining & quarrying | 2019 |
| Sky Sirens Studio Ltd | Arts & recreation | 2023 |
| Sky Vector Limited | Administrative & support | 2020 |
| Sky Zero Group Ltd | Construction | 2026 |
| Skyconic Limited | Construction | 2023 |
| Skyheating.London Ltd | Construction | 2021 |
| Skyland Property Invest Ltd | Real estate | 2022 |
| Skyline Facade Design Limited | Professional & scientific | 2020 |
| Skymatz Resources Limited | Administrative & support | 2024 |
| SL Cambridge House LLP | | 2021 |
| SL Heating & Plumbing Limited | Construction | 2017 |
| Slade Dental Practice Limited | Health & social care | 2026 |
| Slav Limited | Construction | 2008 |
| Slaymark Horizons Limited | Real estate | 2014 |
| Slda Ltd | Real estate | 2025 |
| Sleeper & Sons Contractors Limited | Construction | 2009 |
| Sleeper & Sons Holdings Limited | Real estate | 2014 |
| Sleeper Construction Limited | Construction | 2025 |
| Slick Management UK Ltd | Other services | 2022 |
| Slidaway Limited | Construction | 2002 |
| Slide Tribe Limited | Wholesale & retail | 2018 |
| Slimmer Meals Ltd | Manufacturing | 2025 |
| SLK & SON Limited | Real estate | 2017 |
| Slok Convenience Store Ltd | Wholesale & retail | 2021 |
| Slotpatch Property Management Limited | Households | 1991 |
| Slotspan Property Management Limited | Households | 1990 |