Sue,
Not chosen my xmas hol yet but did Costa Brava a couple of years ago and really enjoyed.
Heathrow is big but can be easy, you have two options to get to and from your hotel either hoppa buses or on a service bus (easier than it sounds depending on which hotel).
How are you getting to heathrow ?
Where do you arrive ?
Which hotel ?
With that info some of us can offer advice i would think.
Mick