Havent read whole thread but on the breastfeeding thing, I had a hilarious thing with my friend when our little ones were babies. She's black with a white DH and her son is very white, with blonde hair; I'm blonde with a brown-skinned, dark-haired mixed-race daughter. You guessed it - someone actually asked if we were breastfeeding each other's babies!!! I have to say we laughed ourselves stupid.
I get the 'Are they yours?' and 'Are you the nanny?' stuff all the time - used to give me a pang, not so much now but I'm afraid I am still always glad when the girls call me mummy within people's earshot. Worse for my friend who now has one white and one brown baby - she has actually been challenged about them by strangers, which is very upsetting.
Mum2Boys3 - I get that 'Where's their dad from?' stuff too, which I always hesitate over because the true answer is 'Tooting' but I know that won't satisfy! His mum is, and his late dad was, Jamaican - do I answer their question literally, or go into family history with passing strangers?!
There is also a very definite lingering assumption that you are a single mum ... oh, the awful stereotyping of black men ... When DD1 was very young, a black man asked me 'Is the dad still around?' - it took me a while to realise what he meant but he actually went on to say a load of stuff about 'Well you make sure he sticks by you' etc etc - for goodness sake I wear a wedding ring and have been with their dad for a decade
Oh and on the geographic differences: all the above occurred in central London.