How to deal with money in espana Watch
Where are you going in Spain?
Weigh it up - you'll be charged any way you look at it to convert £ to €, so you might as well carry the wad of cash around. Just don't flash it in public!
Yes, you get charged to change from £ to Euro (or any other currency). And yes, you get charged (varying amounts, depending on which bank you have an account with) to withdraw from ATMS. So either way, you have to pay. Agreed.
What I don't agree with (and this is just my opinion, not saying yours is wrong) is the statement that therefore you "might as well carry cash around." If you get pickpocketed/mugged (unlikely, but it does happen) then you've lost all of it. Debit cards are replaceable (even while abroad). Cash isn't.
I don't even bother getting any Euros before going to Spain now (I go regularly, as I have family there). I rely entirely on using ATMs or paying on debit. They are everywhere, and I always keep another (debit or credit) card safe somewhere as backup in case mine gets lost of stolen.
There are a few countries in the world which are cash-only economies and where it's impossible/very difficult to withdraw money from ATMs or use debit cards. They are often a pain in the neck to travel in due to the need to carry cash around (sometimes huge wads of it) and the constant worry about what happens if it gets lost/stolen/blown away in the wind. For the vast majority of countries I'd always recommend relying mostly on debit cards, perhaps taking a bit of cash along as an emergency. Safer and more convenient.