| Silver Three Projects Ltd | Professional & scientific | 2021 |
| Silverback Internet Services Limited | Information & communication | 2010 |
| Silverdale Management Company Limited | Households | 1990 |
| Silverglory Ltd | Real estate | 1998 |
| Silverline Management Solutions Ltd | Information & communication | 2013 |
| Silverman Consulting Ltd | Information & communication | 2016 |
| Silvers Management Limited | Households | 1987 |
| Silverspoon Links Ltd | Administrative & support | 2020 |
| Silverstone Mews (Maidenhead) Management Co. Limited | Households | 2003 |
| Silvertown Holdings Limited | Construction | 2023 |
| Silverwing Holdings Limited | Financial services | 2021 |
| SIM Building Solutions Ltd | Construction | 2024 |
| SIM UK Legal Ltd | Professional & scientific | 2024 |
| Simbithi Capital Ltd | Financial services | 2022 |
| Simec Holdings Limited | Financial services | 2026 |
| Simec Security Limited | Administrative & support | 2007 |
| Simeones Star Club Ltd | Wholesale & retail | 2025 |
| Simera Limited | Information & communication | 2001 |
| Simex Child Care Ltd | Health & social care | 2023 |
| Simi's Food Limited | Wholesale & retail | 2025 |
| Simis Limited | Information & communication | 1988 |
| Simmons Brothers LLP | | 2008 |
| Simon Builders Ltd | Construction | 2023 |
| Simon Theobald Fine Art Limited | Extraterritorial | 2012 |
| Simon Theobald Limited | Administrative & support | 2012 |
| Simone’S Solutions Ltd | Professional & scientific | 2024 |
| Simple Floors Group Ltd | Construction | 2024 |
| Simple Housing Solutions Ltd | Real estate | 2020 |
| Simple Storage Limited | Transport & logistics | 2019 |
| Simplemarsh (Addlestone) Management Limited | Households | 2010 |
| Simplex Building & Maintenance Limited | Construction | 2018 |
| Simplex Business Solutions Ltd | Professional & scientific | 2001 |
| Simplicity Ideas Limited | Information & communication | 2011 |
| Simplified Education Limited | Education | 2009 |
| Simply Best Properties Ltd | Real estate | 2021 |
| Simply D'Na Limited | Professional & scientific | 1988 |
| Simply Kids UK Ltd | Health & social care | 2017 |
| Simply La'Ren Ltd | Other services | 2024 |
| Simply Laura Ltd | Wholesale & retail | 2026 |
| Simply Lunch Ltd | Manufacturing | 1995 |
| Simply MTD Ltd | Information & communication | 2022 |
| Simply Property Solutions Limited | Administrative & support | 2025 |
| Simply Reliable Cleaning Solutions Limited | Administrative & support | 2024 |
| Simply Safari London Ltd | Administrative & support | 2023 |
| Simply Sins Ltd | Information & communication | 2023 |
| Simply South Hams Limited | Real estate | 2014 |
| Simply South Hams Riverview Limited | Real estate | 2018 |
| Simplyfitnessuk Ltd | Health & social care | 2020 |
| Simpson Building Works Ltd | Construction | 2015 |
| Simtax Consulting Limited | Professional & scientific | 2015 |
| SIN Bar Ltd | Other services | 2018 |
| Sinatrid Studios Ltd | Information & communication | 2021 |
| Sinclair & Brewster Limited | Professional & scientific | 2008 |
| Sinclair Facilities Management Ltd | Construction | 2021 |
| Sinclair Grove Management Company Limited | Households | 2008 |
| Sindh Properties Ltd | Real estate | 2020 |
| Sindher's Pizza Shop Limited | Hospitality | 2025 |
| Singh's Plaice Ltd | Hospitality | 2019 |
| Singhbury's News Ltd | Wholesale & retail | 2016 |
| Single Dads Matter Limited | Health & social care | 2016 |
| Single OR Double Events Limited | Wholesale & retail | 2013 |
| Singleton Hill Management Company Limited | Households | 2002 |
| Sinnathamby Sisters Ltd | Wholesale & retail | 2026 |
| Sinnott Services Limited | Real estate | 2000 |
| Sino Transport Medical (UK) Limited | Manufacturing | 2024 |
| Siphano Picture Books Limited | Information & communication | 1999 |
| Sips & Spirits Ltd | Wholesale & retail | 2026 |
| Sips&Bites Ltd | Wholesale & retail | 2024 |
| Sirene Commercial Limited | Real estate | 2025 |
| Sirius Accounting Services Limited | Information & communication | 2012 |
| Sirius Contracts Ltd | Administrative & support | 2007 |
| Sisi Amor Ltd | Real estate | 2026 |
| Sissako Services Limited | Construction | 2025 |
| Sistas Kitchen Ltd | Hospitality | 2026 |
| Site Assist Advisory Ltd | Real estate | 2024 |
| Site Global Event Logistics Ltd | Transport & logistics | 2024 |
| Sitebox Limited | Construction | 2007 |
| Situ Properties Limited | Construction | 2020 |
| Siva Computers Ltd | Other services | 2020 |
| Sivalaya Constructions Limited | Construction | 2025 |
| Sivar Ltd | Wholesale & retail | 2025 |
| SIX Instruments Limited | Information & communication | 2025 |
| SIX Points Developments (Croydon) Ltd | Real estate | 2015 |
| SIX Rivers Limited | Wholesale & retail | 2014 |
| Sixteen Cranley Gardens RTM Company Limited | Households | 2015 |
| Sixty-Six Gloucester Terrace (Management) Limited | Households | 1966 |
| Siyana Properties Limited | Real estate | 2024 |
| Siyas Enterprise Ltd | Wholesale & retail | 2022 |
| Sizzlers Ice Cream Ltd | Hospitality | 2025 |
| Sizzlers Peri Peri Croydon Limited | Hospitality | 2025 |
| SJ Construction (ENG) Ltd | Construction | 2022 |
| SJ Creativz & Miscellanea Ltd | Information & communication | 2018 |
| SJ Food Mart Ltd | Wholesale & retail | 2019 |
| SJ Properties Enterprise Limited | Real estate | 2017 |
| SJA Store Limited | Wholesale & retail | 2018 |
| SJB London Projects Ltd | Other services | 2026 |
| Sjbeautyclinic Limited | Other services | 2022 |
| SJC Consulting (2017) Ltd | Professional & scientific | 2017 |
| SJC Group Holdings Ltd | Professional & scientific | 2024 |
| SJC Property Developments Ltd | Real estate | 2016 |