My understanding is that if Spain decides to / is pushed out of the euro and reverts back to peseta's, that the peseta will undergo a devaluation, which is something that can't happen within the exchange mechanism. Because the peseta will be devalued it would probably, lose value against Sterling. Therefore your pound would in effect buy more peseta's then it currently does with euro. This is basically what has been happening with Sterling/Euro in the UK over the last 2-3 years, devaluing its currency against the world leading ones by printing money, basically increasing what is in circulation.
I know there is a lot more to it than that, but that it my simplified understanding of the situation.