Irene, I'm happy you started this thread because it prompted yet another outpour of the YLF wisdom. There is so much each one of us can take from this thread - so thank you.
I thought about you a lot over the past few days. One thing that I wanted to add is that you've been at the job for only three days at the time of starting the discussion. Isn't it maybe too early to know the job is not for you? It may be just initial reaction that this is not exactly what you imagined and also natural resistance to the change of what you know as your usual/typical work environment?
I am by no means saying you should not dream big, nor I think I have to spell it out for you. But, for now I think you could think in terms of adapting. Give it time. Let it be. Honestly, this may be your once in a lifetime chance to observe yourself in an unfamiliar circumstances and the learning experience you could not have predicted. It is true, what does not break us, makes us stronger. How - well, it will only become obvious to you sometimes in the future.
Think of this as unlikely, but useful, investment in your career. There jobs and then there are jobs, everyone you come across in the future should appreciate a person who is not afraid of getting on with things. You made a brave move, then you made the best of the circumstances you've found yourself in - while keeping an eye on your actual goal.
One more thing. You mention you could go back to Barcelona and get a better paid job any time. This is something that gives certain weight to how you feel about things. The way I interpret it - deep down, you know there is a rather safe Plan B if things don't work out in London. You know even now that you don't have to do what you do where you are, but can do what's more aligned with your qualifications and abilities - although somewhere else. It's possibly an inner conflict. Perhaps you feel like you're settling for less unnecessarily. I'm saying this partly from personal experience, my husband and I moved a few times in the past, but we almost always had an option if things did not work out as planned. This changes things a lot.
You have every right to feel unsatisfied, but give it some time. I agree you should not do any soul-sucking job, but I believe there is so much you can take out from this. At least you can now compare two different scenarios and how you feel about them: being where you are with no job you want or any job at all, or being where you are and doing something/anything while searching for what you want.