San Sebastian airport is in Fuenterrabía (Hondarribia). It is a minor airport located close to the French border. Owing to its proximity to France and the sea, the runways are rather small so commercial flights hardly ever land on them, excepting domestic ones arriving from Madrid and Barcelona, and some European flights during tourist season.
It is most common to get to San Sebastian from the UK by flying to Biarritz (France) or to Bilbao. Both cities are relatively near to San Sebastian and you can easily go from the mentioned airports to the city centre either by bus or car. Low-cost companies like Easyjet and Ryanair connect Bilbao and Biarritz with the UK at economical prices.
How to get to San Sebastian from: