Dropping out of UCL Economics to do Physics?? :S

Hi I currently have a place to study Economics at UCL. I have started the course but have had serious doubts about whether in fact I would rather be doing Physics and taking a gap year. Tomorrow is the deadline for making my decision about whether to go ahead with it otherwise I will have to fork out £4500.
Basically I've been really unsure about Physics or Economics all along and I am still feeling very unsure, but have maybe been leaning towards the idea of doing Physics. I also maybe feel I would like a gap year so that does not bother me.
I've got 4A*s in Economics Maths Physics and Chemistry so I assume my chances would at least be ok for next year.
I realise that UCL Economics is a very competitive and a very good course. I am still not 100% sure about wanting to do Physics over Economics anyway so would it be a bad idea to throw such a good place away?
I also would probably want to apply to Cambridge for NatSci if I went down the Physics route, so I know the option of taking a year out to think about it is'nt really there as my application would have to be in in just over a week :s
How good would a Physics course from say Manchester (where I feel most interested in) look compared to UCL Economics which I know is regarded as a very good course and I feel lucky to have it, if I was going into a non-Physics related field?
Thanks so much for any help.