I also have Infinity and HH5 and share much of your story,
I have been with BT fibre since it came to our area. at ;east 12 months back now.. I live about 10 yards from the cabinet and have enjoyed the full throt 80meg down and 20meg up (very important to me is the up, as I edit/publish 4K/60fps video that is of 65-75Mbps bitrate, obviously not wanting to wait hours and hours to upload a 8GB file for something like the Utube....
that is, until about 4 weeks ago... since then, our BT fibre has been NOTHING but trouble. There semed to be outages throughout are area with people I know on BT and checking online, it kept reporting that there were problems, although it rarely (if it was even mentioned)....
Since this point, and after days of intermittent internet, and after contacting BT, they slowed our down speed from 80meg to 74meg. Hardly a problem losing 8-9% of my download speed, losing 6meg of my upload speed would be another story.
Since then, we've had days where its has been fine, and others (like now) where it's not working for more than a second to 2, before the router seems to reboot to display every colour of the rainbow in one after anothers sequence.
It has been cut out for AT LEAST 36 hours in the month of may. angry and annoyed i am paying for something that worked great until recently, and now i've had to buy an unlimited topup with a mobile providor that allows creations of hotspots to get by.
get it sorted BT