sounds like rust debris from a corroded steel coolant return pipe behind the engine has blocked the radiator (hot upper pipe, stone cold lower pipe, overheating).
coolant or bubbles continuously gushing over the expansion tank, especially when revving, could be blown HG (letting exhaust force coolant out of the system) as a result of warpage from overheating.
rad fan always on and gauge always red because the sensors in the head say the circulating coolant is HOT and the rad ain't cooling it.
my advise is
-drain coolant,
-disconnect both upper & lower pipes,
-remove the radiator, tilt upside down and flush it backwards to unclog it till the water gushes out freely. peep inside the top hose port to check inside the radiator is clear. look through the outside fins against the sky and check most of the core is clear/unblocked.
-remove the thermostat and check it opens/closes at 82C
[MEDIA=youtube]WZlLH_qfrpU[/MEDIA]
-while your there, remove both temperature sensors and test their ohms/degC are within this graph
https://www.micra.org.uk/threads/pollymobiles-rebuild.35251/page-119#post-704079
-flush the block with a garden hose in the thermostat housing and draining out the coolant return pipe
-reassemble, fill with coolant
-leave cap open, start engine
-bleed system by squeezing the top hose to shake out bubbles, keep the radiator brimmed, transfer excess to the reservoir if needed, till the thermostat fully opens at 82C and the fan engages twice. close cap and keep reservoir at max level.
