I am worried they will (a) not let me board my flight from Edinburgh-Seville;
There are no exit checks when leaving the UK. Airline staff will check your passport to see if you are allowed to enter Spain. They are extremely unlikely to check for an entry stamp. They will just want to see the ID page.
(b) detain me or worse deport me (to the US) from Seville;
Spanish border authorities don’t care about how you entered the UK. They won’t look for a UK entry stamp.
(c) not let me back in the UK at the end of the trip.
A British citizen cannot be denied entry to the UK. It may take some time, but even a photo of your UK passport ID page, or your UK passport number will help establish your identity. You are a British citizen, so you will not be deported.
Credit:stackoverflow.com‘
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