| Sigdev Limited | Information & communication | 2016 |
| Sightline AI Limited | Information & communication | 2025 |
| Sigma V Ltd | Information & communication | 2019 |
| Signaloid Limited | Professional & scientific | 2019 |
| Signalsmith Audio Ltd | Information & communication | 2020 |
| Signapse Ltd | Professional & scientific | 2022 |
| Signature Environmental Solutions Ltd | Construction | 2023 |
| Signature Management Ltd | Professional & scientific | 2021 |
| Signature Oak Buildings Limited | Construction | 2012 |
| Signature UK Group Ltd | Transport & logistics | 2023 |
| Signet Court Investments Limited | Real estate | 2005 |
| Signet Court Management Limited | Households | 1992 |
| Signet Estate Management Ltd | Real estate | 2026 |
| Signetics Ltd | Manufacturing | 2025 |
| Signorellis Deli Ltd | Wholesale & retail | 2016 |
| Signwork Ltd | Manufacturing | 2004 |
| Sikora Property Ltd | Real estate | 2021 |
| Silhouette Solutions Limited | Information & communication | 2000 |
| Silhouette Studios Limited | Professional & scientific | 2025 |
| Silicon Frog Ltd | Professional & scientific | 2022 |
| Silicon Insights Limited | Professional & scientific | 2023 |
| Silicon Microgravity Limited | Professional & scientific | 2014 |
| Silicon Verification Consultants Ltd | Information & communication | 2022 |
| Silk Cinematics Ltd | Information & communication | 2026 |
| Siloxa Limited | Real estate | 2017 |
| Silva Facilities Ltd | Administrative & support | 2026 |
| Silva Investments Ltd | Real estate | 2017 |
| Silvaclass Limited | Real estate | 2022 |
| Silver Acorn Estates Limited | Real estate | 2015 |
| Silver Jeans Trading Co., Ltd | Manufacturing | 2024 |
| Silver Street (Reading) Management Company Limited | Households | 2002 |
| Silver Wise Investment Ltd | Real estate | 2021 |
| Silver Wolf Partners Limited | Financial services | 2025 |
| Silvercam Limited | Real estate | 2015 |
| Silvercrust Limited | Real estate | 2019 |
| Silverlink Consulting Ltd | Other services | 2023 |
| Silverlink Property Ltd | Real estate | 2019 |
| Silvermans Office Solutions Limited | Wholesale & retail | 2003 |
| Silvermore Consultancy Limited | Professional & scientific | 2026 |
| Silverscore Productions Limited | Arts & recreation | 2017 |
| Silverstar Maritime Co., Ltd | Manufacturing | 2017 |
| Silvertie International Co., Ltd | Extraterritorial | 2020 |
| Silverwell Energy Limited | Professional & scientific | 2000 |
| Silverwell Holdings Limited | Financial services | 2017 |
| Silverwell Technology Limited | Professional & scientific | 2000 |
| SIM International (UK) | Other services | 1958 |
| Simar Consultancy Limited | Professional & scientific | 2022 |
| Simarc Property Management Limited | Real estate | 1985 |
| Simba HPC Limited | Information & communication | 2007 |
| Simba Motorsport Ltd | Manufacturing | 2020 |
| Simeon's Trustees | Other services | 1960 |
| Simian Studios Ltd | Information & communication | 2025 |
| Simitar Limited | Professional & scientific | 2009 |
| Simmonds Interactive Limited | Information & communication | 2005 |
| Simmons Innovations Limited | Professional & scientific | 2014 |
| Simmz Group Ltd | Real estate | 2026 |
| Simology Limited | Construction | 2024 |
| Simon Cavell Ltd | Real estate | 2024 |
| Simperly Events Ltd | Administrative & support | 2006 |
| Simpers Marquee Hire Limited | Other services | 2011 |
| Simpers Personalised Clothing Limited | Administrative & support | 2016 |
| Simple Property Management & Services Ltd | Households | 2018 |
| Simpletech Innovations Ltd | Wholesale & retail | 2025 |
| Simplify IP Limited | Professional & scientific | 2022 |
| Simply Bikes Ltd | Wholesale & retail | 2025 |
| Simply CFO Ltd | Professional & scientific | 2026 |
| Simply Consulting Limited | Information & communication | 2001 |
| Simply Dolomiti Limited | Administrative & support | 2007 |
| Simply IOT Ltd | Information & communication | 2020 |
| Simply Technologies(Edtech) Ltd | Education | 2025 |
| Simply Thatch Ltd | Construction | 2018 |
| Simplybestcare Ltd | Health & social care | 2018 |
| Simprints Technology Limited | Information & communication | 2014 |
| Simpson Capital Investments Limited | Financial services | 2019 |
| Simpsons Nurseries Limited | Wholesale & retail | 2007 |
| Simpsons of Newmarket Limited | Wholesale & retail | 2014 |
| Simsage (UK) Limited | Information & communication | 2019 |
| Simsage Group Limited | Information & communication | 2021 |
| Simula Ltd | Hospitality | 2022 |
| Simux Defence Ltd | Information & communication | 2025 |
| SIN CRU | Health & social care | 2020 |
| Sina Health Ventures Ltd | Information & communication | 2025 |
| Since 9/11 | Professional & scientific | 2010 |
| Sincere Group Limited | Wholesale & retail | 2026 |
| Sincerity New Material Co., Ltd | Manufacturing | 2023 |
| Sincerity Personnel Limited | Administrative & support | 2013 |
| Sine Industry Ltd | Manufacturing | 2023 |
| Sinet Creative Limited | Information & communication | 2012 |
| Sinet Estates Limited | Real estate | 2022 |
| Singer Tradition Ltd | Education | 2025 |
| Singers Abroad Ltd | Administrative & support | 2018 |
| Singh Clothing Goods Limited | Wholesale & retail | 2025 |
| Singh INT UK Ltd | Wholesale & retail | 2025 |
| Singledge Management Limited | Households | 2018 |
| Singleetai Limited | Real estate | 2020 |
| Singularis Limited | Information & communication | 2000 |
| Singz Co. Ltd | Real estate | 2021 |
| Sino Performer Industrials Co., Ltd | Extraterritorial | 2019 |
| Sino Pluse Ltd | Wholesale & retail | 2023 |
| Sinoeu Line (UK) Limited | Transport & logistics | 2017 |