Friends of mine got married in Spain last year with a simple civil ceremony and it took them a lot of hard work arranging all the paperwork. It’s not easy so a lot of people in Southern Spain marry in Gibraltar. Of course your girlfriend (fiancee?) being Spanish might make it easier. This link discusses the formalities:
http://www.weddingchaos.co.uk/wedding-abroad/spanish-wedding.asp
Another couple I know are marrying in a registry office in Britain and then exchanging vows at a ceremony here.
Whatever you decide to do, best wishes.
