For example to Australia or Canada specifically, what would you recommend an 18 year old studies at university, or a specific career path to achieve this. This is without taking into account specific interests, where the only purpose is to gain skills to emigrate?
Pick a subject you both like and are fantastic at (world level great) then check university links from the UK institution to the country you want to live in.
Get onto the exchange programme. Study there. Look for post-grad options in that country leading to either local job at senior level intake, or PhD and post-doc work there.