As above, earth and (ecological/conservation oriented) biological science courses will involve more fieldwork, which may well involve travelling internationally. Engineering in general, especially chemical, may as well involve site visits and so on. Of course this may not actually require you to be living abroad - it's entirely possible to live and work in an office/university in the UK but periodically go on field trips and site visits abroad in the course of this work.
However generally in any STEM area you can reasonably look for jobs abroad - these are normally the "skilled worker" areas other countries are looking to sponsor visas for, so you won't be wanting for opportunities if you get a good first degree and can demonstrate good credentials in your area of expertise (e.g. higher degrees, especially getting your PhD, but beyond that at least relevant work in fieldwork/dissertation/thesis/project modules/papers/opportunities). At the moment you should focus on just pursuing the subject(s) you enjoy the most, doing well in them, pursuing any relevant projects beyond the usual curriculum as and when the opportunity arises (internships, placements, summer research projects, etc, etc) and once you are nearing the "finish line" so to speak, come back to this and see what might be possible.