1) London...then take Ryanair to Santiago (then bus to Sarria via Lugo)
2) London, then EasyJet from London (Stanstedt) to Oviedo (airport called Asturias), then train (http://www.renfe.es) to Lugo and bus/taxi to Sarria.
3) Barcelona, then take Vueling (Vueling.com) to Santiago, and bus from there to Sarria.
4) Madrid and take bus/train to Lugo and bus/taxi to Sarria... or get a cheap flight from Madrid to Santiago (see http://www.spanair.comhttp://www.iberia.comhttp://www.aireuropa.com )
5) Porto is an option, I think you would have to go via Santiago to get to Sarria.