You can get all of the free to air channels without a Sky subscription, you just need a dish and a sat box (a Sky box without a card will do, but isn't ideal as I'll mention later).
The free to air channels include BBC, ITV etc, but not Sky Sports, Sky Movies, Nat Geo, Dave etc. If you want to know what channels you wil get then just pull the viewing card out of your Sky box and see what you get.
In Spain you'll need a larger dish than in the UK, somewhere around 1.5 to 1.8 metres. It will need clear line of sight to the sattelite which is South South East of La Marina. If you want to watch one channel while you record another then you'll need a Twin LNB on the dish and twin cables from the dish to the box. If you're likely to want another box i another took (probably without a Sky card) then think about this when you have the dish installed and the cables run, as a twin LNB won't be sufficient in that case.
If you just want the free to air channels then you can use an old Sky box without a viewing card, but you won't be able to hide the unwanted channels (the subscription channels, plus any others that you're not interested in) and you won't be able to pause live TV or record with this setup.
There are a variety of Chinese made sat boxes that are in wide use in Spain that you could use instead of a Sky box, but my advice would be not to bother, as the user interface is venerally not very nice.
We use a Humax HDR-1000s that gives us the Freesat channels and has a nice user interface. It allows you to record, pause live TV, hide the subscription channels etc. We find that the free to air channels are enough to keep us entertained while we're in Spain, although in the UK we do have a Sky subscription. I think we tend to watch less TV while we're in Spain (we spend about half the year ther, in 6-8 week chunks).
There are systems that allow you to watch all the Sky channels without having a Sky subscription. These are obvioulsy illegal and require a special box and a small (compared to Sky) annual subscription. I've seen one of Thesein use and the quality was excellent, but we are happy with the Freesat solution.
If you have an internet connection and want to use UK catch-up TV then you can, but you may need to use a Smart DNS service such as Smart DNS Proxy:
This masks your IP address, so it appears that you're in the UK.
Hope this helps,