I suspect each subject and University is slightly different.
DS was offered PhD funding at the LSE along with guaranteed teaching, which unlike in the US would have been paid in addition. (The LSE is always on the hunt for Masters and PhD students who can hack the maths/stats and are willing to work as teaching assistants on their very large UG economics course, though in practice the Masters course was so intense that it would have been an unwelcome distraction) Once we factored in the tax free element and the guaranteed teaching DS would have been on quite a significant salary, though, after advice, opted to go to the US.
There was also plentiful paid Research Assistant work, which DS did from the end of his second UG year. Tutoring to London's private sixth formers will also pay. From the experience of friends, Imperial's Physics PhDs were doing well. DS, had he been more entrepreneurial, could also have picked up work tutoring affluent first and second year students who needed to pass their compulsory maths courses.
He was lucky that his subject was in high demand. Other LSE Masters programmes are a lot cheaper but the scope for additional paid work will be much more limited.
100 PhDs sounds like a lot. DS was one of an intake of 30 on a six year programme, though a number have already been poached by banks etc. He got away with Research Assistant rather than Teaching Assistant work in his early years, though got some additional paid work covering for a friend in the (well regarded) Business School teaching some MBA students. None were natural econometricians, and he hated being the youngest in the room, but their catering was great. (Not surprising given they are paying about $250k for the two year programme. DS and his friends seem to live off MBA leftovers.) DS rejected the idea of tutoring members of the football team, which was additional money and which I thought would be interesting. College athletes, even though recruited for their sports skills, need to maintain a certain GPA to be eligible for the team so the University is very keen for them to pass Econ101.
When he applied he did not know how competitive funded PhD places were. It is only after seeing some of his friends struggle to find places that he realises he was very lucky indeed. It is almost certainly different in other subjects, but it seems rare for the LSE to recruit staff who do not have overseas, usually US, experience.