Where are you flying back to? That could well determine which airport you need easy access to.
Living in the UK, I've found it easiest to use Bilbao up until Carrion de los Condes. After that it's been Madrid or Santiago de Compostela or A Coruna. I like to fly into Heathrow or Bristol if I can.
From Carrion de los Condes I took a bus to Palencia and then a train to Burgos (though a bus would have been better as the train station is quite far out of town). From Burgos it's an easy 2 hours bus ride to Bilbao.
If you can't get as far as Carrion, then Fromista has a station and might be the next best option to get to Bilbao or Madrid.
Do some research using Rome2Rio.com and look at all your options.