Oops !
Irene it seems you have misunderstood my reply. I did, carefully say 'Why do us Brits.....' However, I will not now tempt fate by suggesting, in reply to your question, what you will need to do, in case I offend again.
My post was to inform those, who did not know, that changing one's name on marriage is not a legal requirement in UK and certainly not in Spain.
And to inform, that if one wishes to change their name (in UK or Rep of Ireland) they can so do without any form of 'legal process' or expense.
I know some people (women usually) get 'lumbered' with a surname which they dislike, even hate, and don't realise that it was a matter of choice: They could have avoided it, whilst still marrying their true love.