Sounds to me like you want to, and would be happiest at UCL. You're right - it is an incredibly significant part of your life, so you "loved it" why are you asking this question?!
But my thoughts on the topic are:
- Imperial: academics
- Bristol: academics & social life
- UCL: social life
I know that's a very basic breakdown, but I think, ultimately that's the kind of trade off you would probably be making (when I say social life, I just mean that the universities are more social, and when I say academic, I meant the universities are known to produce high-calibre graduates). And on the subject of international reputation, (I'm being ignorant here) but what purpose does it serve unless you want to work abroad?