| Sifcon International PLC | Wholesale & retail | 1946 |
| Sigma Guarding Limited | Administrative & support | 2005 |
| Sign M25 Ltd | Other services | 2024 |
| Signed by MAI Ltd | Other services | 2020 |
| Silver Beauty Products Ltd | Wholesale & retail | 2025 |
| Silver Maple Holdings Limited | Financial services | 2023 |
| Silvercrest Pheonix Limited | Other services | 2020 |
| Silversun Ltd | Wholesale & retail | 2025 |
| SIM Hub Ltd | Arts & recreation | 2023 |
| Simal Transport Limited Ltd | Transport & logistics | 2025 |
| Simi Group Ltd | Construction | 2022 |
| Simion Tutors Limited | Education | 2025 |
| Simms Logistics Limited | Transport & logistics | 2023 |
| Simpl Accountants and Business Advisory Limited | Professional & scientific | 2012 |
| Sinco Properties Limited | Real estate | 2014 |
| Singh & Khalsa Medicare Ltd | Other services | 2016 |
| SIP & Savor Ltd | Hospitality | 2026 |
| Sirdar Rectory Ltd | Households | 2026 |
| Sireen Healthcare Limited | Health & social care | 2013 |
| Sirho Estates Ltd | Real estate | 2025 |
| Sirho Ltd | Other services | 2023 |
| Sirina Limited | Health & social care | 2019 |
| Sissy LEA Ltd | Arts & recreation | 2021 |
| SITE14 Ltd | Real estate | 2025 |
| Siteterm Property Management Limited | Households | 1990 |
| Siva Estates Ltd | Real estate | 2018 |
| SJM East Anglia Limited | Financial services | 2021 |
| SK Elite Construction Ltd | Construction | 2026 |
| Sketchy Bones Collective Ltd | Manufacturing | 2020 |
| Skevy & Co Accounting Limited | Professional & scientific | 2025 |
| SKG Const & Maintenance Ltd | Construction | 2024 |
| Skilled Tech Limited | Information & communication | 2020 |
| Skillet Hill Farm (Holdings) Ltd | Financial services | 2014 |
| Skillet Hill Farm Limited | Administrative & support | 2014 |
| Skillmatches Esport Limited | Arts & recreation | 2018 |
| Skills Addict Ltd | Education | 2024 |
| Skills Development Board Ltd | Other services | 2013 |
| Skinbydone Limited | Other services | 2025 |
| Skipjack Music Limited | Information & communication | 2009 |
| Sky Atlas Limited | Transport & logistics | 2025 |
| Skyblue Global Trading Ltd | Wholesale & retail | 2020 |
| Skylines.Jewellery Ltd | Wholesale & retail | 2019 |
| Skyrocket Digital Therapeutics Ltd | Manufacturing | 2018 |
| Skyrocket Phytopharma (UK) Limited | Manufacturing | 2012 |
| SLE Security Ltd | Administrative & support | 2026 |
| Sloan Company Secretarial Services Limited | Administrative & support | 2018 |
| Slyujskh Ltd | Wholesale & retail | 2025 |
| SM & KA Property Group Ltd | Real estate | 2021 |
| SM Dannys Limited | Other services | 2015 |
| SM Social Care Ltd | Health & social care | 2022 |
| Smart 7 Ltd | Information & communication | 2020 |
| Smart Building Services London Limited | Construction | 2016 |
| Smart Life Investments Limited | Real estate | 2025 |
| Smart Life Leases Limited | Real estate | 2025 |
| Smart Security and Electrical Limited | Construction | 2021 |
| Smartsource Builders Limited | Construction | 2024 |
| Smartways Properties Ltd | Construction | 2024 |
| SMG Automation Limited | Wholesale & retail | 2024 |
| SMI Placings Limited | Financial services | 2003 |
| Smillie Holdings Limited | Real estate | 2019 |
| Smith & Gomez Ltd | Real estate | 2022 |
| Smith and Tucker Farms Limited | Agriculture & fishing | 1970 |
| Smith Distribution Limited | Wholesale & retail | 2025 |
| Smith Distrubtion Ltd | Wholesale & retail | 2025 |
| Smith's Property Group Services Ltd | Real estate | 2025 |
| Smitul Properties Ltd | Real estate | 2023 |
| Smoking Hippo Limited | Information & communication | 2010 |
| SNG Railings Ltd | Construction | 2025 |
| SO Near Productions Ltd | Information & communication | 2025 |
| Soft Armor Ltd | Manufacturing | 2025 |
| Softaware Commerce Ltd | Information & communication | 2020 |
| Sojeva Ltd | Information & communication | 2021 |
| Soji Prop Ltd | Information & communication | 2019 |
| Solaire Training & Consultancy Ltd | Professional & scientific | 2022 |
| Solarease Ltd | Construction | 2025 |
| Solmaz Catering Ltd | Hospitality | 2022 |
| Solstar Property Ltd | Real estate | 2024 |
| Solstice Limited | Wholesale & retail | 2017 |
| Solution 4 Everyone Limited | Manufacturing | 2018 |
| Solutions Euro Team (SET) Limited | Professional & scientific | 2010 |
| Somi Services Ltd | Wholesale & retail | 2019 |
| Songlings Ltd | Information & communication | 2026 |
| Sonia & Sons Ltd | Financial services | 2025 |
| Sonik Properties Limited | Real estate | 2023 |
| Sonoworld Diagnostic Services Limited | Health & social care | 2012 |
| Sonson Media Limited | Professional & scientific | 2022 |
| Sopa Holdings Limited | Real estate | 2022 |
| Sophia Petrides Consulting Ltd | Professional & scientific | 2018 |
| Sophie Barker Limited | Information & communication | 2005 |
| Soraceiling Construction Ltd | Construction | 2022 |
| Sorelle Cafe Ltd | Hospitality | 2026 |
| Soren Scientific Solutions Ltd | Professional & scientific | 2023 |
| Sorian Ltd | Transport & logistics | 2025 |
| Sotilleo UK Consulting Ltd | Financial services | 2022 |
| Sound Licensing Limited | Professional & scientific | 2011 |
| South Bucks Rugbytots LLP | | 2023 |
| South View Scaffolding Ltd | Construction | 2018 |
| Southall Management Company Limited | Real estate | 2022 |
| Southern Estate Properties Limited | Financial services | 2020 |
| Southgrove Limited | Construction | 2006 |