There are the odd few cabinets (in enabled areas) BT seem to forget about.
The fact your lines are underground is not important to the provision of FTTC (Infinity is just BT's own brand for a product offered by many other ISPs).
Something like 60% of our town has underground cables.
BT don't seem to be doing cabinets which feed mainly commercial/industrial zones (because Infinity is so cheap, compared with BTs fibre leased lines).
The cabinet (one number higher than ours) at the eastern end of this estate is one BT has forgotten about (not good news for the people living there as line lengths to the exchange means they'll be stuck on around 4Mbps until BT remember they put a new cabinet in 6 years ago and add it to the roll-out).
vDSL2 FTTC Infinity with BT
DL Sync 40Mbps
UL Sync 10Mbps