The lockable window security restrictor has a 20cm long cable, which when installed correctly will limit the opening of the window to under 100mm. Not only will this prevent children from being able to fit through the gap, but it also allows enough air to circulate around a room.
eBay
Durable Sliding Windows Restrictor Windproof Brace Stopper Children Safety Locks Window Support Opener Fixed Angle Controller
Aliexpress UK
Description:[Anti-wear Pad] Window and door locks, 4 pieces per set, with matching anti-wear pads to prevent direct contact with windows and doors[Easy to Assemble] with matching screws - mark the position of the mounting holes with a pen before installation, drill holes in the marked positions, and then fix the lock with screws. When installing, the window needs to be closed to confirm the gap between the window frame and the sash, and gaskets and anti-friction pads should be selected according to the size of the gap.[Easy to Operate] Snap-on lock, more convenient to use. When using the window anti-theft lock, you only need to turn it to lock or release the window sash, prevent children from opening the window[Application Range] The flat lock blocker is sui.
eBay
Free Punch Window Lock Limiter Windows Handle Restrictor Lock Anti Falling Guard Locking Buckle High Rise Child Protection Locks
Aliexpress UK
Upgrade the security on your UPVC window or door. Handle：Plastic Enhance security of UPVC windows and doors. Locks Material: Zinc alloy. 10 x Window Lock （with screws）.
eBay
Secure T Shaped Lock Strong Lock Durable Lock Sashes Blocker Window Jammer Simple Installation for UPVC Doors & Windows
Aliexpress UK
16 Self-drilling fixing screws. 4 cable limiter.
eBay
White Replacement UPVC Window Handles Left/ Hand Zinc Alloy Window Hardware Door Handle
Aliexpress UK
Mila Sprung Child Safety Window Restrictor. Mila Sprung Window Restrictor. 1 x Sprung Restrictor. Window Restrictor. Aluminium Door Hardware. Finger Operated Shoot Bolts. MPL Shoot Bolts. Window Handle Keys.
eBay
Neoteck 2Pcs Window Handle Universal Espag Key Locking Lockable Double Glazing UPVC Window Handle - 30mm Spindle Length
Aliexpress UK
An excellent 2 handed operation window restrictor that has been designed to avoid accidental disengaging. There are no keys required and the device automatically re-engages when the window is closed.
eBay
High quality Aluminium alloy T-lock, steel flat open doors and windows locks Replacement Security UPVC Window Door Sash lock
Aliexpress UK
This Key Locking Window Cable Restrictor Safety Lock is a high-quality product made from plastic, stainless steel, and steel. FUNCTION: The key locking window restrictor is designed to limit the opening distance of windows, enhancing safety and security.
eBay
Children Safety Lock Window Restrictor Windproof Brace Stopper Sliding Cabinet Door Security Sash Locks for Child
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
Restrictor Sliding Wind Brace Telescopic Window Support Stainless Steel Limiter Angle Controller Stopper Heavy Duty Gusset Fixed
Aliexpress UK