| Cassiopeiauk Limited | Sandy | 2023 |
| Cassl. | London | 1986 |
| Cassmic Ltd. | Glasgow | 2016 |
| Casspa Ltd | Tonbridge | 2021 |
| Cassy Marketing Communications Limited | Sheffield | 1989 |
| Cassyannes Ltd | Northampton | 2026 |
| Cast (BFD) Ltd | Bradford | 2005 |
| Cast HR Solutions Limited | Sawbridgeworth | 2025 |
| Cast Metals Federation | Glasgow | 2001 |
| Cast PP Limited | Liverpool | 2016 |
| CAST1617 Limited | Buxton | 2016 |
| Castalia Lounge Ltd | London | 2022 |
| Castaruma Ltd | Chester | 2024 |
| Castaway Spa and Beauty Ltd | Newry | 2018 |
| Castella Contracting Ltd | Bedford | 2018 |
| Castellan Services Ltd | Twickenham | 2026 |
| Castellers of London | London | 2016 |
| Castelli Milano 1938 Limited | Kent | 1994 |
| Castelnau Barber Ltd | Morden | 2020 |
| Castelnau Capital Limited | London | 2025 |
| Castiels Ladies Hair Limited | London | 2011 |
| Castillo Enterprise Limited | London | 2026 |
| Castillo House Limited | Stevenage | 2004 |
| Castle & Green Limited | Chelmsford | 2025 |
| Castle and Co Housing Ltd | Birmingham | 2022 |
| Castle Barber Ltd | Lincoln | 2020 |
| Castle Barbers (Edgeley) Limited | Rossendale | 2020 |
| Castle Beauty Limited | Manchester | 2019 |
| Castle Best Barbers Limited | London | 2018 |
| Castle Bidco Limited | Southampton | 2024 |
| Castle Bites Limited | Leeds | 2019 |
| Castle Building Projects Limited | Chalfont St. Giles | 2018 |
| Castle Care 24/7 Ltd | Edinburgh | 2018 |
| Castle Co NW Limited | Liverpool | 2017 |
| Castle Commercial Cleaning Limited | Oldham | 2013 |
| Castle Commercial EPC Ltd | St. Asaph | 2023 |
| Castle Communication Services (Yorkshire) Ltd | Wakefield | 2021 |
| Castle Computing Solutions Ltd | Whitby | 2025 |
| Castle Court Freehold Limited | Cardiff | 2004 |
| Castle Croft CUT Ltd | Wolverhampton | 2024 |
| Castle Dawn Limited | London | 2026 |
| Castle Douglas Hairdressers Limited | Castle Douglas | 2016 |
| Castle Embroidering Limited | Leicester | 2001 |
| Castle EPC Limited | St. Asaph | 2019 |
| Castle Estates (South Coast and Weald) Limited | East Sussex | 2003 |
| Castle Force Limited | Ossett | 2018 |
| Castle Garden Construction Limited | Gerrards Cross | 2015 |
| Castle Glow Ltd | Birmingham | 2025 |
| Castle Groundworks Ltd | Irvine | 2017 |
| Castle Hairdressing Limited | Guildford | 2005 |
| Castle Hill Advisors Limited | London | 2017 |
| Castle Hill Insolvency Ltd | Newton Abbot | 2017 |
| Castle Hill Regeneration Group | Dungannon | 2014 |
| Castle Hill Tyres Limited | Axminster | 2016 |
| Castle Home Solutions (Dudley ) Ltd | Dudley | 2025 |
| Castle House Management Company (Leighton Buzzard) Limited | Leighton Buzzard | 2001 |
| Castle Hvac Limited | Altrincham | 2013 |
| Castle Lane DRY Cleaners Ltd | Bournemouth | 2023 |
| Castle Lane Nails Spa Limited | Bournemouth | 2013 |
| Castle Linton Ltd | Cowdenbeath | 2018 |
| Castle Midco 1 Limited | Southampton | 2024 |
| Castle Midco 2 Limited | Southampton | 2024 |
| Castle Nails 6 Ltd | Cardiff | 2023 |
| Castle Nails Bar Limited | Stratford | 2019 |
| Castle Nails Ltd | Pembroke | 2019 |
| Castle of Auchry Windfarm (Holdings) Limited | Aberdeen | 2021 |
| Castle Place Barbers Limited | London | 2019 |
| Castle Point GAS & Heating Co. Limited | Benfleet | 1983 |
| Castle Portable LOO Hire Ltd | Maidstone | 2020 |
| Castle Process Limited | Aberdeen | 2011 |
| Castle Refrigeration Service Limited | Paddock Wood | 2020 |
| Castle Road Community Association Community Interest Company | Portsmouth | 2024 |
| Castle Rock Landscapes Limited | Bracknell | 2019 |
| Castle Skin Clinic Limited | Halstead | 2017 |
| Castle Spa Lucy Ltd | Brough | 2010 |
| Castle Spa Sheena Ltd | York | 2022 |
| Castle ST Barber Ltd | Cambridge | 2025 |
| Castle ST Blow DRY Bar Limited | Lisburn | 2014 |
| Castle Street Carpets Limited | Stockport | 2023 |
| Castle Street Hairdressers Ltd | London | 2022 |
| Castle Tanning Limited | Dungannon | 2026 |
| Castle Topco Limited | Southampton | 2024 |
| Castle Valeting Services Ltd | Nuneaton | 2021 |
| CASTLE786 Food Limited | Ilford | 2022 |
| Castlebuild Limited | Manchester | 2018 |
| Castleburn Services Limited | London | 2024 |
| Castleburn Solutions Limited | Kintore | 2011 |
| Castlebury Limited | Essex | 2022 |
| Castlebury Services Limited | London | 2026 |
| Castlecap Investments Limited | Chichester | 2011 |
| Castlecourt Nails & Spa Ltd | Belfast | 2024 |
| Castlecroft Chippy Ltd | Birmingham | 2015 |
| Castlecroft Domestic Services Limited | Wolverhampton | 2003 |
| Castlederg Christian Fellowship | Castlederg | 2022 |
| Castlefield Basement Limited | Worcester | 2026 |
| Castlefield Beauty Bar Ltd | Halifax | 2015 |
| Castlefield Property Services Limited | Bury | 2025 |
| Castleford Boat Yard Limited | Castleford | 2014 |
| Castleford Hand Car Wash Limited | Castleford | 2020 |
| Castleford Heritage Trust | Castleford | 2004 |