it is only possible to count periods from other EU-countries according to the EU-rules.
It follows from EU-regulation No 883/2004 on the coordination of social security systems that the rules can only be used by EU nationals. This means that for nationals from outside the EU/EEA/Switzerland, only stays in Norway, Sweden, Finland and Iceland can be counted (equals residence in Denmark). Third-country nationals can thus not include residence in other EU/EEA countries.
That stays in the nordic countries can be included follows from the Nordic convention about social security.
In the nordic convention the nordic countries has agreed that also non-EU nationals who moves between the nordic countries must be treated like EU-nationals when it comes to social security coordination. Thats why non-EU nationals still can count periods with stay in the Nordic countries (but not from other EU/EEA countries).
So it is not relevant if Brexit has some consequences, because as a non-EU citizen, you can not include stays from other than the nordic countries.
Anders Weber, www.a-kasser.dk