My SO and I prepared basically everything so that we could live in either country (e.g. we got our qualifications recognised, learning the community language etc.). For us, the deciding factors were the following:
careers. I work as a teacher, and earn great money here in Switzerland. On my salary, I can easily take care of my partner and me. Since my partner is an engineer, he also earns good money. If I moved to England, my salary would decrease a lot.
my health. NHS is okay, but if you have a chronic illness like me, it's not great. Furthermore, with my salary decrease, I wouldn't be able to go private. Health care is expensive in Switzerland, but I do get the care I need.
families. My partner has a huge family, but isn't overly close to them, my family is small, and we're very close.
education systems. As a teacher, I'm not a fan of the British school system (private vs. public schools, standardised testing, neglecting foreign languages etc.).
lifestyle. My partner wants dogs, and loves hiking.....and many of his interests can be lived better here in Switzerland than London, where he's from.