Lady Penelope is correct in that an OC doesn’t require a by-law to enter a lot to install window locks as the legislation already allows for the OC to carry out work to common property, and owners must allow access. Window locks would be common property as they are attached to the windows which are common property in most schemes.
If you aren’t happy with a window installers opinion, simply go to another one. As Lady Penelope pointed out, it may be an idea to source a compliant lock yourselves then engage a lock smith to install them.