Hi there,
We are going through the process at the moment.
1. You need to be aware, if not already, that there are differences in the registration.
2. You are only 'fiscally register'able' if you reside for more than 183 days in a year - therefore making your home in Spain your permanent residence and therefore obliged to declare incomes etc ( which would include CGT) wherever earned.
We have sought the services of a couple of the 'advisers' but they seem to have conflicting advice and therefore we have decided to let our solicitors in Spain advise us, prepare and collate all the necessary paperwork and then make the appointments for us. Their advise is to apply for the 'new' TIE and then, if we do stay for the majority of the year at our home in Camposol, to declare as mentioned.
You probably know that part of the application for residency is the 'padron'. And when applying for the 'padron', amongst all .the paperwork you will need, will be a tenancy agreement or your property deeds (escritura).
If you don't manage to get the application for 'TIE' in the system before 31/12/20, then, unless a miracle happens with Brexit terms, Spain will treat you exactly the same as someone from Canada, USA or anywhere else on this planet. The terms for residency will then be more severe in terms of income, investment, health cover etc.
Hope this helps