| Signisoft Technology Solutions Limited | Information & communication | 2010 |
| Signs Unlimited Warrington Ltd | Manufacturing | 2013 |
| Signum Estates Limited | Real estate | 2018 |
| Sigo Group Limited | Financial services | 2026 |
| Sigraph Creative Studio Ltd | Professional & scientific | 2022 |
| Siki Consulting Ltd | Professional & scientific | 2024 |
| SIL.2 Limited | Health & social care | 2014 |
| Silcock Properties Ltd | Real estate | 2018 |
| Silhs Global Services Limited | Administrative & support | 2025 |
| Silk Estates 2024 Ltd | Real estate | 2024 |
| Silsila Digital Ltd | Professional & scientific | 2019 |
| Silv Consulting Ltd | Information & communication | 2014 |
| Silvastia Ltd | Other services | 2018 |
| Silver Birch Planning Services Ltd | Construction | 2021 |
| Silver Fern Engineering Services Ltd | Professional & scientific | 2025 |
| Silver Rain ML Limited | Professional & scientific | 2024 |
| Silver Sisters Limited | Manufacturing | 2023 |
| Silver-Line Sitting Service Ltd | Health & social care | 2025 |
| Silverbrook Tech Limited | Administrative & support | 2025 |
| Silverbyte Solutions Ltd | Wholesale & retail | 2025 |
| Silverhill Manor Residents Management Company Limited | Households | 2025 |
| Silverock Partnership Ltd | Administrative & support | 2024 |
| Silversands Properties Limited | Real estate | 2000 |
| Silverstar Limited | Information & communication | 2021 |
| Silverway Corporate Ltd | Administrative & support | 2012 |
| Silvio Aleksandrov Ltd | Transport & logistics | 2022 |
| SIM Developments NE Limited | Real estate | 2023 |
| SIM Dynamics Limited | Other services | 2024 |
| Sima Installations NW Ltd | Construction | 2025 |
| Simath-Tech Ltd | Professional & scientific | 2020 |
| Simfonik Ltd | Information & communication | 2024 |
| Simi 5 Ltd | Construction | 2020 |
| Simkiss Control Systems Limited | Manufacturing | 1999 |
| Simkiss Group (SCS) Limited | Financial services | 2022 |
| Simmons Lifting Solutions Ltd | Construction | 2017 |
| Simmons Process Engineering Limited | Professional & scientific | 2018 |
| Simmy Chahal Digital Consultancy Ltd | Information & communication | 2019 |
| Simon & SON Property Management Ltd | Real estate | 2019 |
| Simon Clarke Consultancy Limited | Professional & scientific | 1995 |
| Simon Education Limited | Education | 2021 |
| Simon FX Trading Ltd | Other services | 2024 |
| Simon Hogg Inarchitecture Limited | Administrative & support | 2016 |
| Simon Mason Designs Ltd | Professional & scientific | 2002 |
| Simon Stephenson & SON Limited | Transport & logistics | 2025 |
| Simon Wilson Consulting Ltd | Information & communication | 2025 |
| Simple Lets Group Limited | Real estate | 2021 |
| Simple Solutions Financial Management Ltd | Financial services | 2009 |
| Simple Technology UK Limited | Wholesale & retail | 1991 |
| Simplify Insights Ltd | Information & communication | 2019 |
| Simplify Technovation Ltd | Administrative & support | 2021 |
| Simply Scissors Limited | Wholesale & retail | 2011 |
| Simply Surveying Limited | Administrative & support | 2016 |
| Simply Transformation Limited | Professional & scientific | 2021 |
| SIMPLYMARIA25 Ltd | Health & social care | 2015 |
| Simpson Tax Consultancy Limited | Professional & scientific | 2013 |
| Simtek Design Ltd | Professional & scientific | 2012 |
| SIN Holdings Ltd | Professional & scientific | 2025 |
| Sinaia Market Ltd | Wholesale & retail | 2024 |
| Sincere Health Limited | Health & social care | 2024 |
| Sinclair Consulting Counselling and Training Limited | Health & social care | 2024 |
| Sinclair Electrical NW Limited | Construction | 2019 |
| Sindi Ltd | Other services | 2017 |
| Siney's Cleaning Crew Ltd | Administrative & support | 2024 |
| Sinfoniqa Ltd | Information & communication | 2013 |
| Singleton Creative Ltd | Information & communication | 2021 |
| Singleton Design Solutions Limited | Administrative & support | 2009 |
| Singleton Solutions Ltd | Professional & scientific | 2020 |
| Singletons Group Limited | Manufacturing | 2024 |
| Sinth Leather Limited | Manufacturing | 2025 |
| Siraj Consulting Ltd | Information & communication | 2011 |
| Siri Services Limited | Administrative & support | 2024 |
| Sirion Limited | Wholesale & retail | 2025 |
| Sirius Informatics Ltd | Administrative & support | 2026 |
| Sirocco Consulting Ltd | Construction | 2023 |
| Sirpur Engineering Services Limited | Extraterritorial | 2024 |
| Sirron Contracts Limited | Administrative & support | 2012 |
| Sirwan Barber Ltd | Other services | 2023 |
| SIS Projects Limited | Construction | 2003 |
| Sisman Energy Consultancy Ltd | Administrative & support | 2014 |
| Sismy Ltd | Transport & logistics | 2016 |
| Sistem Solutions Ltd | Professional & scientific | 2023 |
| Sister Co Studio Ltd | Other services | 2025 |
| Sisu Safety Ltd | Administrative & support | 2026 |
| Site Found Ltd | Information & communication | 2026 |
| Site Investigation Specialists Ltd | Professional & scientific | 2019 |
| Site Project Cleaning Ltd | Construction | 2018 |
| Site Project Services Ltd | Construction | 2009 |
| Sitec Infrastructure Services Ltd | Information & communication | 2007 |
| Sitemix Direct Limited | Manufacturing | 2024 |
| Sither Consultancy Limited | Information & communication | 2022 |
| Sither Limited | Real estate | 2018 |
| Sivasai Consultancy Limited | Administrative & support | 2009 |
| Siverek 63 Ltd | Hospitality | 2025 |
| Sivner Consulting Limited | Professional & scientific | 2019 |
| SIW Engineering & Construction Limited | Administrative & support | 2011 |
| SIX Twenty Three Ltd | Administrative & support | 2023 |
| Sixteen 3 (Holdings) Limited | Professional & scientific | 2011 |
| SIXTEEN3 Ltd | Manufacturing | 2009 |
| Size Consultants Ltd | Professional & scientific | 2021 |
| SJ Bain Limited | Administrative & support | 2016 |