You're in debt. You have £500 left after paying your rent, bills and debt each month. Does that include groceries, living costs, clothes, visits to family abroad etc?
I suppose "borrowing" off your parents isn't quite the same - but is more debt the answer? What if they need the money back?
You need another £700pm just to pay for childcare - and then live on what exactly?
You can't give up your job - or even take anything but the minimum maternity leave because you need to be earning £18400 gross pa to meet your husband's visa requirements. Which means you can't claim WTC.
Great idea for your DH to be a SAHD, but a word of warning - it's not everyone's cup of tea.
If you move to a cheaper area then you will have the cost of commuting which will account for most of what you will save on accommodation. Tell me a cheap area where you can move to within cheap and easy commuting distance of London?
I'm sure you both love your jobs but if you've chosen a low paid career and worked for 16 years, it's a shame you didn't start thinking about this 10 years ago and saved some money.
You have no assets, only debts, and an uncertain future. Everything depends on you working full time with no drop in earnings or loss of employment.
I really can't see how you will realistically manage the additional expense of a baby in these circumstances.
Another poster mentioned the MSE website - you might want to take a look at the budget planner.
You might also want to have a look at StepChange with regard to handling your existing debt.
Another good site relating directly to the cost of having a baby in the first year only (it doesn't get cheaper after that, either - quite the reverse) is The Money Advice Service.
But then, after all that, I notice you've said I am encouraging DH to take a banking job in the city ! He's not keen!.
Is that right? He could take a well paid job, get you out of debt and out of this mess, carve out a career to assist his visa application - and he's not keen?
I hope that was a joke!