| Simon Cooper Consultants Limited | Professional & scientific | 2000 |
| Simon Johnson & Sons Builders Limited | Construction | 2015 |
| Simon Lincoln Recruitment Solutions Limited | Administrative & support | 2009 |
| Simplycollectibles Ltd | Wholesale & retail | 2024 |
| Simpson Land and Property Limited | Professional & scientific | 2016 |
| Sinfiltrate Limited | Manufacturing | 2010 |
| Sips of Joy Ltd | Wholesale & retail | 2023 |
| Siri Bespoke Garden Rooms Ltd | Construction | 2023 |
| Sirio Limited | Professional & scientific | 2025 |
| Siska Construction Ltd | Construction | 2019 |
| Sisters Women Business Ministry | Other services | 2018 |
| Site Bites Limited | Hospitality | 2025 |
| Site Supplies Panda Ltd | Wholesale & retail | 2024 |
| Sitetech Engineering Solutions Ltd | Construction | 2024 |
| Sitwell Dearden Ltd | Professional & scientific | 2023 |
| Siva Consultancy Ltd | Professional & scientific | 2024 |
| Sivank Ltd | Households | 2019 |
| SIXTY6SOCIAL Ltd | Professional & scientific | 2022 |
| Siyanbola Property Investments Ltd | Real estate | 2022 |
| Sizco Solutions Limited | Information & communication | 2014 |
| Sizzlers Welwyn Garden City Ltd | Hospitality | 2013 |
| SJ Foods Limited | Wholesale & retail | 2024 |
| SJ Mellish Accountancy Services Ltd | Professional & scientific | 2011 |
| SJ Rivers Consulting Limited | Other services | 2018 |
| SJA Social Ltd | Professional & scientific | 2024 |
| SJB Demolition Ltd | Construction | 2014 |
| SJB Group & Holdings Ltd | Financial services | 2016 |
| SJB Plant Hire Limited | Administrative & support | 2001 |
| SJB Residential Ltd | Real estate | 2017 |
| SJC3 Limited Limited | Hospitality | 2024 |
| SJH Motorsport Limited | Wholesale & retail | 2017 |
| Sjjj Couriers Ltd | Transport & logistics | 2020 |
| SJK Building Services Ltd | Construction | 2017 |
| SK Transfer Ltd | Transport & logistics | 2025 |
| Skaar Ltd | Wholesale & retail | 2025 |
| Skala Europe UK Limited | Wholesale & retail | 2025 |
| Skandapurvaja Ltd | Hospitality | 2026 |
| Skateistan UK | Education | 2012 |
| Sketch Agencies Europe Limited | Wholesale & retail | 2024 |
| SKI Definition Limited | Administrative & support | 2017 |
| Skills Synergy Ltd | Education | 2023 |
| Skinetica Scientific Skincare Ltd | Wholesale & retail | 2022 |
| SKJ Food and Wine Limited | Hospitality | 2015 |
| Skjaldmaer Holdings Limited | Wholesale & retail | 2022 |
| SKS & Partners Limited | Professional & scientific | 2015 |
| SKS Soft Ltd | Information & communication | 2011 |
| Sksi Items Ltd | Information & communication | 2026 |
| Sky Digital Subscription Management Limited | Professional & scientific | 2017 |
| Skyehurst Limited | Professional & scientific | 2025 |
| Skyhighscaffoldingservices Limited | Construction | 2017 |
| Skyline Construct Ltd | Construction | 2018 |
| SL Traders Solution Ltd | Professional & scientific | 2026 |
| Slabside Limited | Construction | 2024 |
| Slaters Property Services Ltd | Real estate | 2021 |
| Slavov DRY Lining Ltd | Construction | 2017 |
| Slcooling Ltd | Construction | 2025 |
| Sleater HR Limited | Administrative & support | 2023 |
| Sleepy Frog Wellness Group Ltd | Other services | 2025 |
| Slims Workwear Ltd | Wholesale & retail | 2026 |
| Slink Health Limited | Arts & recreation | 2011 |
| Sljcivil&Structural Engineers Ltd | Construction | 2018 |
| Slough Online Community Interest Company | Information & communication | 2018 |
| SLS Medical Ltd | Wholesale & retail | 2026 |
| SM Capri Ltd | Hospitality | 2025 |
| SM Performance Psychology Ltd | Education | 2020 |
| Sm-Ink Print Limited | Manufacturing | 2018 |
| SMA HR Limited | Administrative & support | 2021 |
| Smart Advise (London) Ltd | Professional & scientific | 2025 |
| Smart Global Software Solutions Limited | Professional & scientific | 2011 |
| Smart Global Trading Limited | Professional & scientific | 2008 |
| Smart Ground Ltd | Other services | 2020 |
| Smart Internet of Things Ltd | Information & communication | 2011 |
| Smart Modular Ltd | Construction | 2026 |
| Smart Platform Rental Limited | Administrative & support | 2000 |
| Smart Sign Solutions Ltd | Manufacturing | 2022 |
| Smart Sustainable Heating Ltd | Construction | 2015 |
| Smart Training Academy Limited | Administrative & support | 2012 |
| Smart Vest 360 Limited | Wholesale & retail | 2025 |
| Smartbase Solutions Ltd | Information & communication | 2003 |
| Smarter Energy Solutions Limited | Professional & scientific | 2017 |
| Smartly Solutions Ltd | Wholesale & retail | 2025 |
| Smartlytics Consultancy Limited | Information & communication | 2016 |
| Smarttsense Connect Ltd | Information & communication | 2016 |
| Smash N Drip Ltd | Hospitality | 2025 |
| Smiler Agency Limited | Manufacturing | 2018 |
| Smilerite Ceramics Ltd | Health & social care | 2011 |
| Smiley Electrical Limited | Construction | 2019 |
| Smith's Coffee Company Limited | Hospitality | 1998 |
| Smiths Detection Group Limited | Professional & scientific | 2004 |
| Smiths Detection Investments Limited | Administrative & support | 2004 |
| Smiths Detection-Watford Limited | Manufacturing | 1950 |
| Smiths Home Builds Limited | Real estate | 2020 |
| Smjg Enterprise Ltd | Information & communication | 2023 |
| Smod Consultants Limited | Information & communication | 2010 |
| Smooth Transformations Ltd | Construction | 2018 |
| SMS Accounting and Business Services Ltd | Professional & scientific | 2020 |
| Smyth QS Services Ltd | Professional & scientific | 2022 |
| Sn&Tk Ltd | Professional & scientific | 2025 |
| Snap Goods Limited | Information & communication | 2025 |
| Snaps Group Limited | Administrative & support | 2023 |