Just to wrap this thread up, and thank you to all that replied.
I decided in the end to just buy the official Tesla Winter Wheel Package for the 2021 Model 3 Performance.
Tesla confirmed yet again, that they do not recommend 18" or 19" wheels for the 2021 Model 3 Performance... and they also confirmed any pre-2021 model 3 - 20" wheels don't fit either.
The correct UK Winter Package is this one... (Part Code : 1643533-00-B)
The Model 3 20” Überturbine Wheel and Winter Tire Package provides maximum safety, performance and control in all winter driving conditions. Improved traction, stability and braking efficiency in temperatures under 45°F. Includes: 4x 20X8.5J Überturbine Wheels 4x 235/35/20 Pirelli WINTER...
shop.tesla.com
In the UK, it's cost me £ 3,588.40 inc VAT (but I can claim the VAT back, as it's a Company purchase), so I'm really paying £ 2,990.33 (with a reduction in Corp Tax, and no Income Tax on the cost).
For that, I get a brand new set of Uberturbine 20" alloy wheels, centre caps, BLE TPMS sensors, Pirelli Sottozero 3 Winter Tyres, balanced & fitted... all ready to just stick them on the car.
I figured in the end, that they can be sold on in the future, and I always have a spare set of genuine Uberturbine wheels to swap about if any get damaged in any way.
Hey ho...