Key Locking Sash Blocker PVC-u Door & Window Restrictor. They're easy to fit and easy to operate too - just a twist of the Blocker Arm will lock or release the sash. The arm can also be locked into place using the key provided.
eBay
12 Inch Durable Windows Restrictor Windproof Brace Stopper Children Safety Locks Window Support
Aliexpress UK
Single Restrictor Kit Includes 1 x Window Security Restrictor. 4 or 8 Window And Door Security Restrictor Kit（s）. When the cable is in place the distance the window can open is limited, by using the key the cable can be removed from one end and the window can be opened fully.
eBay
External Window Lock Buckle Alloy Safety Locks UPVC Door Sash Jammer Security Restrictor Lock With
Aliexpress UK
Child-resistant locking device compliant with BS EN 16281: 2013, ideal for windows and balcony doors. - Compliant with BS EN: 13126-1: 2011 (includes BS EN 13126-5), Grade 3 Impact Resistance—a certified hardware restriction for windows and doors.
eBay
T Shape Window Locks Sash Blocker Jammer Plastic Steel Window Restrictor Lock For Home Window Door
Aliexpress UK
4 x Window And Door Security Restrictor Kit. When the cable is in place the distance the window can open is limited, by using the key the cable can be removed from one end and the window can be opened fully.
eBay
Aluminum Alloy Door Window Security Lock Window Restrictor Locks Sliding Window Lock Child Safety
Aliexpress UK
Suitable for aluminium or UPVC windows. The restrictor is fully concealed once the window is closed.
eBay
Window Safety Lock Window Restrictors Self Adhesive No Drilling Window Locks Child Safety Cable Lock
Aliexpress UK
Concealed UPVC Window Restrictors. Manufactured from stainless steel. Complies to BS6375 : Part 2 : 1987. Ideal for retro-fitting. Available in left hand, right hand or pairs (one right and one left).
eBay
2x Window Security Lock Stainless Steel Window Limiters Child Safety Sliding Window Restrictor for
Aliexpress UK
Winkhaus Centurion OBV restrictor WHITE. Winkhaus Centurion OBV Tilt & Turn Window Restrictor - White. Egress Easy Clean Window Hinges UPVC Friction Stay - 12" x 17mm Stack. Suitable for Tilt & Turn windows.
eBay
Durable Sliding Windows Restrictor Windproof Brace Stopper Children Safety Locks Window Support
Aliexpress UK
PVC UPVC Window Door Security lock Restrictor for Baby Child Safety Cable + Key. Single Restrictor Kit Includes 1 x Window Security Restrictor. 4 or 8 Window And Door Security Restrictor Kit（s）. When the cable is in place the distance the window can open is limited, by using the key the cable can be removed from one end and the window can be opened fully.
eBay
Durable Sliding Windows Restrictor Windproof Brace Stopper Children Safety Locks Window Support
Aliexpress UK