Hi!
I just wanted to share some information that I found out recently on this subject.
It is possible to have dual (British / Spanish) nationality, but apparently only until the age of 18, at which point, if a person wants Spanish nationality, they would have to relinquish their other nationality and swear allegiance to the Spanish State.
The only nationalities to which this doesn?t apply are those of Spanish or Portuguese speaking countries, Andorra, Philippines, Equatorial Guinea, and Portugal.