| Signature Projects Group Limited | Construction | 2025 |
| Significan'T (UK) Limited | Professional & scientific | 2003 |
| Signpost Property Ltd | Real estate | 2025 |
| Signshop (Yorkshire) Limited | Professional & scientific | 2005 |
| Sigtrends Ltd | Professional & scientific | 2021 |
| Sigurro Limited | Transport & logistics | 2020 |
| SIK Medical Services Limited | Health & social care | 2021 |
| Sikasha Organics Ltd | Wholesale & retail | 2026 |
| Sikona UK LLP | | 2018 |
| Silaghi Ltd | Wholesale & retail | 2017 |
| Silecyn Auto Centre Ltd | Wholesale & retail | 2026 |
| Silecyn Capital Ltd | Financial services | 2026 |
| Silent KID Ltd | Professional & scientific | 2024 |
| Silflow Ltd | Real estate | 2021 |
| Silk and Soil Ltd | Wholesale & retail | 2025 |
| Silk Family Law Limited | Professional & scientific | 2010 |
| Silk Letting Ltd | Real estate | 2018 |
| Silk Road Associates Limited | Professional & scientific | 2016 |
| Silk Trader Macc Ltd | Hospitality | 2026 |
| Silkey Ltd | Other services | 2022 |
| Silkstone Consulting Limited | Professional & scientific | 2011 |
| Silo Pizza Ltd | Hospitality | 2026 |
| Silqera Ltd | Other services | 2024 |
| Silsys Automation Ltd | Professional & scientific | 2009 |
| Silvanta Ltd | Health & social care | 2024 |
| Silver FOX Cleaning Services Ltd | Administrative & support | 2023 |
| Silver Jewellery Ltd | Wholesale & retail | 2012 |
| Silver Oak Arboriculture Limited | Agriculture & fishing | 2022 |
| Silver Pride Facilities Management Limited | Administrative & support | 2003 |
| Silver Tablet Ltd | Health & social care | 2013 |
| Silver Transgroup Ltd | Transport & logistics | 2020 |
| Silver Travel Advisor Limited | Information & communication | 2010 |
| Silver Umbrella PAY Ltd | Professional & scientific | 2020 |
| Silverback Enterprises Limited | Arts & recreation | 2019 |
| Silverback Group Limited | Arts & recreation | 2021 |
| Silverback Homes Limited | Real estate | 2021 |
| Silverback Performance Coaching Ltd | Education | 2023 |
| Silverback Pro Supplements Ltd | Administrative & support | 2025 |
| Silverbackx Ltd | Financial services | 2024 |
| Silverdale Properties Limited | Real estate | 2012 |
| Silverdale Resource Ltd | Information & communication | 2003 |
| Silvergate Residential Ltd | Construction | 2018 |
| Silverleaf Sports & Prestige Limited | Wholesale & retail | 2016 |
| Silverleaf Technology Limited | Information & communication | 2002 |
| Silverlight Marketing Limited | Professional & scientific | 2018 |
| Silverline Executive Travels Ltd | Transport & logistics | 2023 |
| Silverline Limited | Transport & logistics | 2026 |
| Silverman UK Jewellers Ltd | Wholesale & retail | 2019 |
| Silversmith Consulting Ltd | Professional & scientific | 2019 |
| Silverstar Leisure Limited | Hospitality | 2014 |
| Silverstone Vehicle Management Limited | Wholesale & retail | 2003 |
| Silverwood Care Ltd | Health & social care | 2022 |
| Silvia BIO Limited | Professional & scientific | 2025 |
| Silviu EMI Ciuraru Limited | Transport & logistics | 2025 |
| SIM C Properties Ltd | Real estate | 2024 |
| Simao Vtech Limited | Information & communication | 2025 |
| Simba Industrial Services Ltd | Hospitality | 2024 |
| Simba Promotions 1 Ltd | Administrative & support | 2023 |
| Simcare Learning Ltd | Education | 2023 |
| Simco-Bands Limited | Real estate | 2018 |
| Simdure Ltd | Professional & scientific | 2020 |
| Sime Darby Property (London) Limited | Professional & scientific | 1972 |
| Sime Limited | Wholesale & retail | 2002 |
| Simeon Brundell Limited | Health & social care | 2010 |
| Simfika Limited | Information & communication | 2022 |
| Simfileshare Ltd | Information & communication | 2018 |
| Simian Mobile Disco Limited | Information & communication | 2017 |
| Simian Music Limited | Information & communication | 2018 |
| Simisker Design Ltd | Information & communication | 2022 |
| Simko Vehicle Maintenance Ltd | Wholesale & retail | 2006 |
| Simmonds BR Renovations Limited | Construction | 2024 |
| Simmons Fulham Limited | Hospitality | 2016 |
| Simmons Putney Limited | Hospitality | 2021 |
| Simmons Street Light Fulham Limited | Hospitality | 2017 |
| Simo Building Solutions Limited | Construction | 2020 |
| Simo PFP and Joinery Limited | Construction | 2025 |
| Simoda Group Limited | Financial services | 2024 |
| Simoda Limited | Information & communication | 2019 |
| Simola Ltd | Hospitality | 2022 |
| Simon Broughton Brickwork Ltd | Construction | 2024 |
| Simon Byrt Ltd | Information & communication | 2011 |
| Simon Cundy Associates Ltd | Other services | 2015 |
| Simon Fraser Engineering Limited | Information & communication | 2009 |
| Simon Goodlad Limited | Information & communication | 2013 |
| Simon Hollis Limited | Professional & scientific | 2011 |
| Simon Hope Management Ltd | Professional & scientific | 2014 |
| Simon James Cars Limited | Wholesale & retail | 2009 |
| Simon Keighley Building Services Ltd | Professional & scientific | 2019 |
| Simon Mann Limited | Health & social care | 2005 |
| Simon Martner Ltd | Professional & scientific | 2012 |
| Simon Smith Legal Services Limited | Professional & scientific | 2018 |
| Simon Trans Ltd | Transport & logistics | 2016 |
| Simona Ionescu Ltd | Health & social care | 2024 |
| Simoncollinsconsulting Limited | Professional & scientific | 2022 |
| Simone Atkins Limited | Health & social care | 2023 |
| Simonic Precision Limited | Manufacturing | 2004 |
| Simontd Fashion Limited | Administrative & support | 2016 |
| Simos Properties Ltd | Real estate | 2025 |
| Simpa Trans Ltd | Transport & logistics | 2022 |
| Simpkin Machin & Company Limited | Construction | 1946 |