I've only skim read some of the thread, so this has probably been covered already.
If the OP goes into a "low level" job such as waitressing/retail etc, she will likely be on a zero hours contract - and will therefore need to make herself available for work prior to rotas being completed.
Waitressing, in particular, will require her to be available evenings - likely from around 4pm, to set up for evening business.
As a London solicitor, is the OP's DH able to reliably pick up the DC's from after school club at (say) 6pm?
I doubt it.
Is the OP's DH prepared to cover his equal share of DC sickness/INSET days/snow days?
Because, on a zero hours contract, the OP's leverage on this will be severely limited.
The hospitality industry will have very little patience with OP taking time off for child-related stuff, so DH will need to step up.
He'll need to be home sharp each day, and he will need to free himself up to be the default parent evenings and weekends.
If the child is sick at school, and the OP is waitressing at an event, will the DH be able to do his fair share of leaving work to pick up the sick child?
If the DH can commit to all this, I can see this arrangement working.
But, excuse me for feeling cynical.