Are you paying for childcare? Because that's the killer cost here. It's not forever, and I still think that £100k is a lot, but our ridiculously expensive childcare system means that during the young-child years, you can be earning very well and taking home very little.
Op - takes home £1833 a month after tax
Sally on £100k - £5500, which is clearly hugely more
OP - £695 rent, lifetime tenancy, zone 1
Sally - £2500 for a small double bed, zone 1. No security and £5k deposit needed
Childcare - op doesn't mention any. But on salary of ~£28k (for £22k take home) she should be getting at least £120ish a month in child benefit, and 30 hours a week if child is nursery age
Sally - likely working full time, and long hours. Despite what pps have said, she couldn't afford a full time nanny, who would be well over £3k a month in central London. She'd be looking at childminders and wraparound childcare, and only 15 free hours either. If she have long after school care and a child at nursery she will easily be paying over £2k a month, which brings her to less than £1k for general living, not so dissimilarly to op.
This is not forever and in time Sally will be much better off - but in the short term, you can see how people can be earning well but feel stretched.