This worked for me:
I travelled from Sydney, arrived at Heathrow, had six hours to get myself to Stansted airport (had a bit of a tough time getting through Heathrow immigration..... think the officer feared I was going to go AWOL while crossing London! I think it would've helped if I'd had proof of a hotel reservation in Sarria).
Then I got a Ryan Air flight to Santiago (they insisted on a hard-copy flight confirmation when I checked in: if people didn't have it, they had to pay extra..... just showing your email booking on your phone wasn't acceptable! Also the queues for the various boarding gates were very chaotic.... I almost got on the plane to Germany instead of Spain! Lots of us were confused!) - but at least Ryan Air is cheap, if you book in advance.
Had a good and quick flight to Santiago, then got a bus from the airport to Lugo (no bookings required, and just pay on the bus), then a shortish wait in Lugo for another company's bus to Sarria (no bookings required; buy your ticket from the ticket in the bus station.) A lovely bus trip.
I arrived in Sarria in the evening, and started walking the next morning. Next time I'd have a spare day in Sarria first.
All the best with your planning.