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I think that the idea is to travel to Piortugal (permitted by UK after May 17th) - walk or bus into Spain - return to Portugal and back to UK. The problem is that at present Portugal wont let UK citizens in!
Whatever route you choose - have an extra day in Baiona to help recover from your journey. There is plenty to see and do there. The Tourist Office is very helpful.
You can go by train. This takes all day with a 4 hour wait in Vigo Return from Santiago has more choice.
DeutchBahn
https://www.bahn.com/en/view/index.shtml
will give the times in English. The Spanish Train Company RENFE runs the trains.
I do not know what long distance buses can do.
This site is run by Ivar at in Santiago de Compostela.
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