If you have searched this, you are in a very large group. It is one of the most asked questions about this city, and the answers you usually get are a listicle of things to do rather than an explanation of why it is hard.
Why London is genuinely harder than other cities
It is enormous, and crossing it costs an evening. The most quoted complaint from Londoners about their own social lives is not loneliness, it is logistics: an hour each way to meet someone for a drink turns a casual coffee into a half day commitment. This is why local matters more here than anywhere else. A friend twenty minutes away will be seen ten times more often than a friend an hour away, regardless of how much you like them.
It is transient. Around two in five Londoners were born outside the UK, private renting is far more common here than nationally, and half of private renters have been in their home under three years. People arrive, and people leave. A very common and rarely admitted feeling is a reluctance to invest in someone who may not be here next year.
There is no default meeting place any more. London lost a lot of pubs, a lot of council run community spaces, and a chunk of the office. Hybrid working is more common here than anywhere else in the country, which is good for your commute and bad for the accidental social contact that used to come free with a job.
The two facts that should change your approach
1. The first year is the hard one
Analysis of the Survey of Londoners found that people who had lived in London less than a year were significantly more likely to be severely lonely. Every band after that, one to two years, two to three, three to five, was not statistically significant. It is a cliff rather than a slope, and it resolves, one way or another, inside about twelve months.
If you are inside that first year, you are in the hardest part and it is not evidence of anything about you. If you are past it and still stuck, the thing to change is the format, not the effort.
2. Repetition beats introductions
Casual friendship takes something like 40 to 60 hours of shared time. Nothing you do at a one-off event can accumulate those hours. This is why people who go to lots of events and meet lots of people can still end up with nobody to call, and it is the single most useful thing to understand about adult friendship.