The green residency certificate, the new TIE residency cards and even a receipt that your residence permit is being processed by the Spanish authorities are all valid travel documents to show your residency in Spain, a British Embassy spokesperson told the Bulletin this morning.
Over the weekend a group of British travellers were turned back at Heathrow airport as a result of problems with their Spanish residency documentation. Only Spanish nationals and British citizens who can show that they are residents in Spain can travel here at the moment as a result of the new COVID restrictions.
The incident at Heathrow airport prompted the Spanish government to publish a letter underlining the fact that the green certificate was still valid.
You can see Spain's official advice on valid documentation here.