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Is it possible for me to change my uni course biology to a completely different one like economics at a different university, like one I haven’t applied to like ucl or kcl
You'd have to reject all of your offers, enter extra, phone up the unis you wish to apply to (UCL and KCL will not be available as they are oversubscribed for those courses and many others) and ask if they would accept a new personal statement that you would send them after you applied, apply to said uni.
It all depends on the uni. Unis can enter and leave extra whenever they want so they can close if they fill up all of their places. Unis can be in extra but not appear in clearing as well so it is hard to know. However, you can probably judge that if a uni is not in extra then they will not be in clearing.