Posted: Thu Oct 29, 2020 3:30pm
Third country nationals are already monitored, and the 90 days in and 180 is taken seriously with fines for overstaying but it is also possible to issue a ban on returning for a certain length of time. As EU nationals we have been privileged with no real checks, which has meant people have overstayed without any consequences. ETIAS comes into operation towards the end of 2021 or early 2022 so that will ensure checks on all non-schengen country nationals entering the schengen area as well. The days of overstaying or visiting too frequently are coming to an end.
Going back to the original post, NIE's can be applied for while in the UK via the Spanish consulate in London or Edinburgh, initially on line and then with an appointment for verification. NIE's are required to open a Spanish bank account, purchase a house or anything that requires a contract. They do not confer any special rights on the holder - except the ability to pay your Spanish taxes!
As others have said, residencia (or rather TIE as non-EU nationals) is only available to those who relocate to Spain and you must firtst qualify under the requirements for income and health insurance.