Most live in flat-shares, which honestly I prefer to living alone anyway. You can easily find central London flat shares for £1000 a month which are walking distance to various central locations, and extremely quick by bus/bike/tube.

I've lived in London for 8 years now as a student (undergaduate masters then PhD) and have lived on around £20k a year (post tax) and it's completely fine tbh. I earn more through extra work but save pretty much all of it.