You don't need the card to access the Free to Air channels that Sky puts out. If you wish to continue getting the Sky pay channels, you will need the card which is matched to your HD box and you will also need a UK address to have the card and bills sent to. Sky will not provide the service in Spain so never phone them if you have a problem or the service will be terminated (it is against their T&C of use).
I used a Sky box at first, without card, simply to get the channels. I changed after a while and bought a HD satellite receiver over here for around €60 with a USB record facility and now get all the channels that freesat offers not just the ones that Sky does (although there's not a great deal of difference). The other benefit is that, during bad weather, if a channel drops out there are plenty of others to choose from which may not be affected. For example, we have ITV 1 HD for London, during the recent heavy storms it dropped out and I chose another ITV 1 HD which was not affected and I couldn't do that on the Sky box. The downside is that there is no EPG on my new box which Sky has.