Getting back to the original question. The actual wording from Fair Trading:
“The safety devices must be able to limit the maximum window opening to 12.5cm, must be robust, and must be childproof. “
So they need to be installed so that they can be engaged when necessary, your windows still can be opened past 12.5 cm if you wish them to at other times.
More here:
We’re having a rather fun time trying to find acceptable solutions with enormous mullioned double hung sash windows circa 1890s here.