You can fly to Santiago de Compostela with the following airlines.
A lot of companies fly to Santiago:
RYANAIR: directly from: Rome, London, Frankfurt (cheap)
AIRBERLIN: London, Zürich, Salzburg, ...
The centre itself isn't too big. You can walk from Harrods in Knightsbridge to St Paul's Cathedral, or even from Oxford Street to Westminster Abbey. If you don't want to walk you can hop on the ...