Hi, StarSurvivor, sorry this is a bit late. Have you taken any action on this?
Speaking as someone who just transferred not only cross department but cross faculty (from engineering to social sciences) at UCL, I will say that it is definitely a possibility, provided you have the necessary subjects at A-level. And that if studying economics is going to make you miserable, you owe it to yourself to switch. I forced myself through two years of a degree I hated because I thought the one I was on originally was more socially acceptable and did a hell of a lot damage to my health in doing so.
That having been said, I would advise waiting until you get to UCL to make the switch. Once you get there, I would let them know as soon as possible (you mentioned you'd like to try economics in case you change your mind; not a bad idea, but listen to instinct, too. If you still think that English is what you want to do, let them know sooner rather than later.), but the process will go a bit more smoothly once you're officially a student. At that point they have your grades, there's no speculation, and you're already a student at UCL; they have less cause to doubt your capabilities and there are processes in place specifically to deal with people who change their mind about their course in their first few weeks.