If you’re going to mandate 10, you might as well mandate 40
I don’t imagine there’s a massive difference in cost between FTTC and bog standard ADSL. Also, don’t forget that BT now own a massive mobile network, so wireless 4G/5G solutions (combined with microwave radio, if sufficient line of site available, or satellite if not) might be more cost effective for the most difficult locations.
Is any obligation going to be placed on major land owners, in terms of allowing equipment on (or under) their land?