The train from L'Aldea is another option. You can get the express for about 12€ to Barcelona Sants and then another to the airport using a T10 is 1€ or if you book early you can get a Talgo for as little as 10€. The Renfe web site is easy to use to check times, prices, book and pay online.
The Talgo takes about 1hr 40 mins, with allocated seat.
The express takes between 1hr 45 and 2hr 15.
Very comfy.
Over 60s can get a gold card for 6€ to get 40% discount on express.
Parking is free and safe.
Beats uk rail service hands down.