| Simmer Dating Ltd | Information & communication | 2025 |
| Simon & Geri Investment Ltd | Real estate | 2024 |
| Simon Church Consulting Ltd | Information & communication | 2019 |
| Simon Cleaning Services Ltd | Administrative & support | 2025 |
| Simon Collard Limited | Administrative & support | 2022 |
| Simon Griffin Antiques Limited | Wholesale & retail | 1985 |
| Simonhenman Ltd | Information & communication | 2022 |
| Simovative Ltd | Information & communication | 2021 |
| Simple Central Vacuum Limited | Manufacturing | 2014 |
| Simple Claims Assistance Limited | Financial services | 2018 |
| Simple Freight Solutions Limited | Transport & logistics | 2017 |
| Simple Group Solutions Limited | Administrative & support | 2025 |
| Simple Knowledge Solutions Limited | Information & communication | 2013 |
| Simple Purity Ltd | Administrative & support | 2022 |
| Simplex Management Solutions Limited | Professional & scientific | 2015 |
| Simplex Properties Limited | Real estate | 2024 |
| Simplicity Financial Planning Ltd | Financial services | 2019 |
| Simplicity Support Services Limited | Administrative & support | 2024 |
| Simplified IT Solutions UK Ltd | Information & communication | 2018 |
| Simplr Creative Ltd | Professional & scientific | 2023 |
| Simply Audit Limited | Professional & scientific | 2022 |
| Simply Essential Couture Ltd | Wholesale & retail | 2023 |
| Simply Good Limited | Hospitality | 2017 |
| Simply Race Limited | Arts & recreation | 2014 |
| Simply Reset Ltd | Administrative & support | 2025 |
| Simply Tree Services Limited | Agriculture & fishing | 2001 |
| Simply Wallet Limited | Financial services | 2024 |
| Simply Wedding Hire Ltd | Arts & recreation | 2019 |
| Simplyhappy Ltd | Manufacturing | 2023 |
| Simprove Property Limited | Real estate | 2020 |
| Simpsons of Greenfield Mill Limited | Manufacturing | 1989 |
| Simrat Carpentry and Renovations Ltd | Construction | 2025 |
| Simta Energy Limited | Wholesale & retail | 2025 |
| Sinai Accounting Ltd | Professional & scientific | 2014 |
| Sinai Gypsum UK Ltd | Wholesale & retail | 2025 |
| Sinapiseed Ltd | Wholesale & retail | 2025 |
| Sinclair Auto Services Ltd | Wholesale & retail | 2017 |
| Sinclair Scaffolding Ltd | Construction | 2019 |
| Sinducare Ltd | Health & social care | 2015 |
| Sinensis Sparkling Tea Ltd | Manufacturing | 2026 |
| Sinewave Communications Limited | Information & communication | 2016 |
| Sinewave Technologies Limited | Information & communication | 2016 |
| Singhequity Limited | Real estate | 2020 |
| Sinnott Engineering Limited | Professional & scientific | 2021 |
| Sinnott Properties Limited | Real estate | 2019 |
| Sinomart. Ltd | Wholesale & retail | 2025 |
| Sintec Electrical Limited | Construction | 2021 |
| SIO Consultants Ltd | Professional & scientific | 2013 |
| SIP Station Lakenheath Ltd | Wholesale & retail | 2026 |
| Sipergy Limited | Wholesale & retail | 2025 |
| Sipro (UK) Ltd | Manufacturing | 2013 |
| Sips & Shades Ltd | Arts & recreation | 2025 |
| Siracusa Show Style Ltd | Other services | 2025 |
| Siraman Holdings Ltd | Manufacturing | 2014 |
| Sirbee Logistics Ltd | Transport & logistics | 2020 |
| Sirfi Food Limited | Hospitality | 2025 |
| Sirhid Holler Consult Ltd | Financial services | 2014 |
| Sirhid Holler Holding Company Limited | Real estate | 2020 |
| Siri Info Limited | Information & communication | 2015 |
| Sirigiri Boyina Properties Ltd | Real estate | 2021 |
| Sirius Housing Group Ltd | Extraterritorial | 2025 |
| Sirius International Holdings Ltd | Financial services | 2026 |
| Sirius Wear Group Ltd | Wholesale & retail | 2021 |
| SIS Cosec Limited | Professional & scientific | 2012 |
| SIS Outside Broadcasts Limited | Information & communication | 2008 |
| SIS Racing Limited | Information & communication | 2023 |
| Sisa Holdings Limited | Financial services | 2018 |
| Sisbet Limited | Professional & scientific | 2006 |
| Siscon Limited | Manufacturing | 2000 |
| Sish Properties Limited | Real estate | 2024 |
| Sisodia Limited | Health & social care | 2020 |
| Sistas Foods Limited | Hospitality | 2026 |
| Sistema Ecodeco UK Limited | Construction | 2004 |
| Site Twin UK Ltd | Professional & scientific | 2026 |
| Sitea Ltd | Transport & logistics | 2019 |
| Sithan Limited | Professional & scientific | 2024 |
| Sithemba Inkosi Ltd | Education | 2026 |
| Sitne & Stanley Holdings Limited | Financial services | 2026 |
| Siva All-In-One Property Service Ltd | Construction | 2026 |
| Sivabalan Associates Limited | Information & communication | 2022 |
| SIW Holdings Limited | Real estate | 2024 |
| Siwel Communications Limited | Information & communication | 2010 |
| SIX Arrows Consultancy Limited | Professional & scientific | 2021 |
| SIX Group Properties Limited | Real estate | 2023 |
| SIX Home Improvements Ltd | Construction | 2023 |
| SIX Lechmere Road Limited | Households | 1995 |
| SIX Mills (Bryngwyn Village) Management Company Limited | Households | 2006 |
| SIX Talents Care Ltd | Health & social care | 2017 |
| Sixes Growth Ltd | Hospitality | 2024 |
| Sixpack Events Ltd | Arts & recreation | 2020 |
| Sixth Wave Food Ltd | Hospitality | 2025 |
| Siya Holdings Ltd | Financial services | 2020 |
| Siya Investments Ltd | Households | 2020 |
| Siza Private Limited | Health & social care | 2023 |
| Sizeless Limited | Wholesale & retail | 2013 |
| SJ Advisory Limited | Professional & scientific | 2015 |
| SJ Nickols Limited | Professional & scientific | 2010 |
| SJ Search Ltd | Administrative & support | 2025 |
| Sj's Purnabrahma Limited | Hospitality | 2025 |
| SJA Hygiene Services Limited | Health & social care | 2012 |