Just to clarify, yours is petrol isn't it? It looks like the front end of a B2x0 but the air intake looks a bit odd!
That aside, if your engine temperature drops once it's warm and you're driving at a reasonable motorway sort of speed, you probably wouldn't notice if the gauge is sticky. Not unless you tapped the dash every so often which can do more harm than good at times.
On the basis that it's a petrol engine, the fan should be controlled by the engine management ECU so no need for a radiator mounted fan switch. Older cars that didn't have the ECU control for the fan used a switch in the rad end tanks as your photo shows.
If the temperature drops as above, the fan wouldn't come on as the engine temperature would be too low, even stuck in traffic. I've got rid of the viscous fan on mine and just use the electric fan (switched by a switch in the same position and mainly fitted because of the air-con) and that rarely comes on, even on hot days.
Going back about a month, maybe 6 weeks to the first hot spell we had, i went down to London to collect some bits. Mostly motorway speeds (i mean proper motorway speeds) but i did get stuck on the M25 about tea-time, still very hot outside and even then the fan didn't come on.
As for what temperature, if memory serves correctly you should have an 88C 'stat in yours so it will be a few degrees above that, probably about 95-98C.
Looking at the switch you linked to, it is a 92-87C rating so it comes on when the temperature reaches 92C and stays on until it has dropped to 87C.
As a rule of thumb, the temperature gauge centre line is about 88C (with 90C being just above) and the start of the gauge is usually about 40C with the top of the gauge usually about 130C.
Please note that apart from the central figure these are approximations and could be slightly different depending on make, model and so on. It's more to give you an idea of what should be happening.
Don't forget that although water boils at 100C in normal atmospheric conditions, because the system is pressurised that raises the boiling point considerably. Most of the caps i know of are 1.1bar/16.5psi/1.1 atmosphere roughly speaking.
There are many ways you can measure the temperature of the coolant, i use an infra-red laser thermometer for convenience, was about £8 from Amazon but i had to wait for the "slow boat from China" for it.
https://www.amazon.co.uk/gp/product/B007NVYWUS
Gone up a bit, now £9.99 and i'm sure if you were willing to pay a little more, you'd get one from the UK, either via Amazon or ebay.
https://www.ebay.co.uk/itm/8-Type-No..._tUiSys6JEUJcQ
One from a UK seller on fleabay there for a couple of quid more.
Pointing it at the top hose or the radiator stub the top hose joins onto will give you the most accurate reading.
Comes in handy for checking a lot of other stuff too - fridge, freezer, central heating radiators (identifies easily if they need bleeding), oven - the list is only ended by your imagination really!
If you get bored checking temperatures, you can shine the laser pointer at the ground and get your cat to chase it too!
All said and done, the first thing to do is to verify what temperature the engine is actually running at before you start chasing (potentially) non-existent sensor faults.
