Hi
You can use Uk currency in Gib. I still use my UK account for salary, paying bills etc. However, it depends where you intend to live. Gib property is expensive to rent but convenient location-wise, Spanish property cheaper but you have the daily commute, language etc. to overcome. Also depends if you have kids of school age as you may not want to put them into the Spanish system if they don't speak Spanish.
There is plenty to do outside of work, it really depends if you like to socialise or engage in sports. The best thing to do is to explore when you get here, there's plenty of choice.
As an aside I used to work in HH, down Perrymount Road - it's a small world...