You'll probably practice expiration dating
This is a transient city, and while we're reliably informed by The Bangles that love is eternal, visas, jobs, and student programs aren't. Sometimes, an accent is both an aphrodisiac and a ticking time bomb, so unless you want to follow them to New York/Sydney/Paris, best prepare yourself.
Living in different zones could be a deal breaker
London is huge. Getting from, say, Clapton Pond to Dulwich can easily take you 90 minutes, and that's a walk of shame most people can't afford.
People are already in relationships... with their careers
This is one of the most competitive cities in the world, and your insane rent doesn't pay itself thanks to all the free time you have. When you’re waking up at 6:30am to squeeze into an overcrowded Tube, working until 8pm, maybe hitting the gym for a bit, and then somehow eating and seeing your friends... who has time for dating?