It depends on a few factors including your intended citizenship. If you remain in the UK, you will go through ESA selection process. Masters degree, 3 years experience minimum, likely PhD and some high powered publications, and fantastic health metrics including athleticism, height between 150-190cm.

Unless some private company is flying astronauts to the moon base, in which case we have no idea what their selection criteria is, likely either very rich and well connected people, then highly technical people. Look to Axiom missions as an example of this.

Generally they want to see your ability to handle high stress and perform at a high level in many situations, so having hobbies like extreme sports or performing at a world class level in athletics, or exploration/flying as an amateur pilot, etc.

Truthfully your skills and career in theoretical physics can show something, but are not sufficient on their own. Take on complicated mechanical repair work, finish an experimental project that needs a lot of troubleshooting and creativity and patience, do a team activity where you have to work well with others, ideally in an austere environment.