In the early years of FTTC all installations were by engineer and all had the stand-alone modem. This was so that Openreach had full control of what was attached to the line whilst they got to know how things would be in bulk usage, rather than small-volume testing.
The same happened with ADSL. It's just that most of us only know about self-install ADSLx.
The router could be anything you chose, and still can be. However, combo modem/routers such as we are used to on ADSLx have to conform to detailed Openreach specifications if you want to be sure of ongoing compatibility as they improve the FTTC technology.
BT Consumer, Sky and TalkTalk all now supply combos that they of course have ensured fit the specifications.
At the start when engineer installs were all you could have, there were two levels of that - in fact there still are. The basic job included checking the modem worked, the more expensive (to the ISP) job included installing the router (BT HomeHub for example).
The indispensable man or woman passes from the scene, and what happens next is more or less the same thing as was happening before.
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