A "W" is what's known as a W-pattern routing, ie, the aircraft flies a route starting and finishing at another base.
Eg, aircraft based at Gatwick flies to Palma, then to Luton (the middle of the "W"), then back to Palma, and finally back to home base at Gatwick.
That way flights from 2 UK airfields to a common destination are accomplished in the same day using 1 aircraft (and most likely 1 crew on this type of routing).