For ground floor and other accessible windows, it is important to fit a sash window lock as well as the standard fastener. There are broadly two types - stops and bolts - and can be key or hand operated. Those with a key operation are usually acceptable for insurance requirements.
Sash stops are fitted to the top sash, usually a few cms above the midrail to prevent the two sashes moving past each other. They allow the window to be opened for ventilation without having to use the lock itself.
Sash bolts hold the window shut and must be removed or retracted to allow it to be opened. This is a more secure option but less convenient for short term ventilation.