Learning from the relatively recent FTTC/FTTP upgrade on my sisters BB remaining with BT as ISP.
https://forum.kitz.co.uk/index.php/topic,26339.msg441623.html#msg441623Make sure you end up with the modem/router where you want/need it to be either by virtue of close proximity to the ONT or having a suitable ethernet inter-connection available especially if you are depending on Wi-Fi coverage as most users do now.
I fully appreciate that you are very likely much more technically informed than my sister or brother-in-law and would be unlikely to end up in a similar situation to the quoted scenario, however, it would appear that once a "working" installation (as tested at the ONT) is completed then it is very difficult (in my sisters case impossible) to get OR (or their designated sub-contractors) to rectify any perceived issues.