Heater tap/valve or its linkage kaput. Hoses collapsed or kinked.
Blocked water system after someone has used RadWeld or similar.
Faulty thermostat more commonly results in over cooling, but stuck
shut can't be ruled out. In the winter at these temperatures the
fan shouldn't cut in very often. If you have acess to a garden hose
then I'd pick a fine dry day and flush the system, with a working
heater tap in the open/hot position. Radiator is easy to remove,
fill with water and shake vigorously, then flush through. Undo
the heater feed hose and see if water goes through it, may be
a gunged up heater matrix. Often the bottom few tubes are
blocked with sediment, they are quite small in cross section,
yours may be mostly or all blocked.

Curious about the auto choke, Uno's I've met, both Mk1 and the
later Mk2 are manual choke, what year is yours?