There are two possible airports to fly to from UK to get to Cabo Roig, Alicante and Murcia ( which is near San Javier )
One of the most useful site for booking flights is which shows the cheapest flights between airports by day.
Directions from the airports to Cabo Roig
Alicante print directions from Alicante airport
Murcia ( at San Javier ) print directions from Murcia airport