Yes, the manual searching at LHR has more or less totally stopped now. The scanner may show a block on the individual which will need inspection, but a visual check is usually good enough. Your mother could perhaps select clothing that would facilitate this if here knee is bound to show up as a block. Other airports have different approaches, but of BA's big stations it's really only LCY that still does frisking to any degree. Scanners are voluntary, however you will be frisked if you decline them. There's no search at LHR if going from ABZ to T5 with BA and going on to somewhere else.

There would be a frisk, incidentally, if your mother is unable to stand unassisted and hold her arms above her head for at least 4 seconds.

You were incorrectly advised regarding the police (though they are the people you approach if you believe an assault took place, security staff are not exempt from the law of the land). The supervisor should take a complaint directly, the CCTV stuff is a bit of a red herring, it doesn't give enough detail to help anyone in this sort of circumstances.