In fact, you can make it really simple for yourself, forget adapters, etc...

Use an piece of plastic (pretty strong) to push down into the earth "hole" in the uk plug socket, then simply push the spanish plug into the remaining two holes of your socket and it will work just fine.

You need to press the earth thing to release the safety mechanism that's in the uk socket, it's a pretty tight fit, but it works fine as long as you don't need to pull the thing in and out of the socket on a regular basis

PS: I will take absolutely NO responsibility if you electrocute yourself.