All academic institutions (universities, academies of science etc.) in the EU can gain access. ELEXIS observers, irrespective of their location, can also gain access.
The institutions must technically maintain a Single Sign-On (SSO) authentication scheme as identity providers (IP) and be affiliated with a national identity federation which is part of the eduGAIN network of identity federations. Institutions without SSO must set it up first in order to gain access.
Does this include countries of the European Economic Area which are not EU members (Iceland, Liechtenstein, Norway) or Switzerland?
Does this include the United Kingdom?
Yes, but only until the effective date of Brexit.
UPDATE (10 Oct 2019) The access is extended till 31 January 2020 to provide extra time for accommodating the not yet clear Brexit result.