I go to Queen Mary, which is the only Russell Group campus university in London, and it's just as you'd expect a university to be in terms of social life. There's an on-campus bar/club called Drapers, and all London unis are a stone's throw from some of the best clubs in the country.
If you don't like the whole nightlife scene, and you're just wondering about making friends in general, then the only way you won't make friends is if you ignore everyone, because everybody in every university around the world is eager to make friends, so there is no way you won't meet people. If you're super shy, then this might prove an issue, but that is a personal issue that you will need to address yourself - it's such a shame when you see a shy person let their insecurities stand in the way of potential friendships!
Those that aren't on a campus (i.e. LSE, UCL, King's, etc.) still have bars, etc. that are for their students, so there are plenty of opportunities to meet people and have fun.
At the end of the day, you are choosing the course because, if you hate your course, you will be miserable, no matter how amazing a social life you might have. Whoever told you it is harder to make friends is either lying or exaggerating!