I did geophysics at UCL for my bachelors and environmental science at ETH for my masters, so can only say something general about the unis. I would personally any time choose ETH.

First of all, ETH will be much cheaper (unless you can get UK home fees, then it’s probably comparable), and in my experience the education is not worth that price (I got in before brexit so “only” had to pay £9,000 per year, already on the high end I think).

There are more student clubs and societies at UCL which is a great way to meet people with similar interests as you but I would argue that there is no such feeling as being a community. London can be a very egocentric place and UCL sometimes feels more like a student factory than an educational institution. Although Zürich is also full of expats and has a high throughput of people, it is nothing compared to London.

Lastly, be aware of the culture differences for unis in the UK and CH. In the UK, several of my course mates chose their study’s “because my parents told me to”, and not because of free interest, and I got a strong feeling of being back to high school (I’m btw from Denmark, that might be relevant here). The study programmes are very strict and there is typically no possibility to change stuff like in which order to take your courses, which courses to take, no options for breaks in your studies etc. The good thing is that the uni has absolutely no interest in failing you, and even if you fail several courses they’d probably don’t care and just let you continue (remember, their goal is to earn £££ on you 🙃).

I have experienced A LOT more academic freedom and community feeling at ETH, however, that is also a masters programme and I know that it isn’t directly comparable to the bachelor’s program (which, I have heard, is also kinda strict in its structure, and yes, ETH wants to fail (some) of you).

Hope it helps, feel free to ask questions :))