People go to university for 4 years to get a Mater's degree in Economics. They then join banks and train for a few more years so that they can try to predict how rates of exchange will vary in the future. Yet all of the research shows that their predictions usually carry more no more weight than pure guesswork! So, if the 'experts' can't tell you which way exchange rates will go, it's unlikely that anyone here can!
However, the pound/euro rate has stayed fairly stable for the past few months and (for what little it's worth) I doubt that there will be any significant changes before September.
To get the best rates, change a small amount of cash (at Marks and Spencer or the Post Office) before travelling, so that you'll have some 'ready cash' when you first arrive. Then use your debit card in Spanish ATMs from there on. The charges you'll face for doing so will almost certainly cost you less than the commission which you'd pay at any exchange facility, either in the UK or in Spain.
Chris