I got into trouble with HSE over the triangular key on panel doors, some doors had the panel key left in the doors, and they said it would be too easy to remove the key from one door and fit it in another, lucky all they required me to do, was to remove all the keys in panels which there was no problem with any employee opening.
I have seen single insulated cables in meter cupboards, and we have on this forum talked about the problems, if an ordinary person, has assess to single insulated cables.
As to if a meter reader has enough training to be allowed access to single insulated cables is up for debate. And the same applies as to if an "in home display" (IHD) and a phone app is enough for a tenant to know how much energy he is using.
I as a retired electrician can't directly read my smart meter, does not really worry me, as I can read the app, and the IHD, and also have other metering devices, so don't really need to read the meter direct, and as to who I would complain to not sure, as British Gas fitted the meter, but I am now with Octopus, but really I should be able to read the meter.
I know where I work we have pad locks in pad locks, as some have my companies key or code, and others have the electric companies key, so either company has assess.
As to if the landlord has been told to change locks or not, we don't know, or why he has changed them, clearly you need to know how much energy you are using, maybe someone has omitted to change the doors to ones with windows in? But we on a forum are really not able to say why the locks have been changed, you need to ask the landlord.