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Good afternoon Maximo, as you have explained due to covid restrictions you could not return this should be a valid reason to be granted pre settled status.
You now will await to hear a decision from the home office.
Thank you again for visiting JustAnswer, please let me know if you need me to clarify anything or if you have any additional questions-I am happy to help!
The letter will be fine to submit . Did you also submit evidence such as articles about restrictions in Spain at that time?
Apart from tenancy agreement, degree certificate and articles this will be sufficient proof.
It was a pleasure assisting you today. Kin