This particular decree does not say anything specific, just adds the requisite of the quarantine. It builds up on a series of previous limitations established by other ministerial decrees produced in the last 2 months.
According to the
first decree that limits movement —a text that has been regularly extended— the limitation does not apply to: nationals and legal residents of the European Union or national and residents of associated states and Andorra if they are transiting. This is were it gets tricky because, while the UK is no longer part of the EU, UK citizens keep enjoying free movement in light of the transitional agreement reached between the EU and Boris that will expire later this year.
So they
should be able to transit. But only transit, not entering the country. The language is quite specific. It says, roughly:
All nationals of a third country will be negated entry into Spain, due to motives of public order or public health, unless they fall within one of the following categories:
a. Residents of the European Union, their spouses or civil partners, if they are transiting to their place of residency.
b. Holders of a long-stay Schengen visa from a member or associated state, transiting to such country.
c. Cross-border workers
d. Health professionals
e. Individuals who work in transportation of goods
f. Active diplomatic or consular staff, members of NGO, of the military or of humanitatian associations.
g. People traveling for reasons of force majeure (family related)
Later it adds:
The entry to Spain will be denied, on the basis of public order or public health, to citizens of the European Union and their family members, if they do not fall within one of the following categories:
a. They are registered as residents in Spain or are in transit to their habitual residence place in another member or associated state or Andorra
So basically, if you are EU, UK, Andorra, yes to transit using the airport, but you will not enter Spain. However, if you are a Spanish resident or national you will be allowed to enter Spain.
And this is where the quarantine decree of today enters in effect: all people entering must quarantine. But the previous decrees establish that only Spanish residents and nationals are allowed in.
Also allowed by this last decree are: cross-border workers, people that transport merchandise, airline and boat crew, and doctors. All of them do not need to quarantine. However, if someone that they have had contact with is detected to be sick they will be forced to do so (think about airline purser in contact with sick traveller, or a Moroccan agricultural worker who has been in contact with a sick co-worker).