In some cases, such as, for example, those of employees, students, eligible relatives of resident persons, choosing the most appropriate residence permit type is quite easy. But in the case of freelancers, entrepreneurs, investors or people having sufficient passive income, the main factor to take into consideration is whether or not they intend to actually live in Portugal.
In case the intention is to merely hold Portugal residency, so as to enjoy visa-free travel in the EU/EEA/Switzerland and eventually qualify for EU permanent residency or citizenship, then the so-called "Golden Visa", Portugal's residency-by-investment programme, is the permit to go for, given its minimal stay requirements (7 days during the first year and 14 days during each subsequent 2-year period).
But for someone who does intend to live in Portugal, it will be faster, more efficient and more cost-effective to apply for a permit other than the Golden Visa, in which case the general minimum stay requirements are nevertheless quite reasonable (4 months during the first year and 16 months during each subsequent 2-year period, provided in either case that no single absence exceeds 6 consecutive months), and may be waived for proven professional or force majeure reasons.
The so-called "Passive Income Visa" is a popular alternative to the "Golden Visa" for someone who intends to live in Portugal and has a sufficient net regular passive income, given that it does not preclude the holder from having a professional activity in Portugal.
Until Brexit becomes fully effective - which will depend on a lot of yet unknown factors and, if not reversed, most probably subject to a relatively long "transition period" - an UK citizen keeps full rights as an EU citizen. Then, even if he ends up losing such rights, if at the time he is a resident of Portugal, he will in all probability keep full rights of residence in Portugal as though he were an EU citizen, and have, so to speak, a "quasi-EU" status, which has already been more or less guaranteed by the EU and the UK.
We cannot, of course, guess the future, but we feel confident that UK citizens who move to Portugal before March 31st 2019 - along with the many thousands of other Portugal-resident Brits - won't face any adverse consequences. It is therefore highly unlikely that, for example, he will be subject to any minimum stay requirements.