Philip I stand corrected... There are a few variations on the EU treaty, the EEA is one of them, the EFTA is the one I meant to address.
EFTA doesn’t require member states to allow free movement, but does allow services across borders. An example of a EFTA member which isn’t in the EU, nor in the EEA is Switzerland. As I understand it, the UK is seeking (remaining) membership of the EFTA and not the EEA, which it indeed left with the Brexit.
However I am not sure whether this is what makes it possible for bunq to continue services in the UK, or not.