Such an interesting thread and topic.
Accents are a huge part of my life.
DMum is from England originally, proper East End cockney accent area. However she didn't live with her family and was taught to speak "properly" by the "auntie" she lived with. She was sent to a fairly posh school in a fairly posh area so sounded fairly posh herself!
Then she emigrated to Canada, met my (Canadian) Dad and had me and my brother. Until I started school, I spoke with an English accent 
I left Canada in 1996, been in England since 2003, and still have 100% of my Canadian accent. I can put on a perfect English "RP" style accent if I want to, but what comes out of my mouth without thinking about it is pure Canadian.
The DDs on the other hand (both of whom were born here) have vair posh accents indeed. Until they get off the plane in Canada, then it takes them about 30 seconds total to lose their English accents. They switch vocabulary too, without even seemingly thinking... pavement/sidewalk, motorway/highway, trousers/pants, rubbish/garbage etc etc.
So I joke around... I've got the passport, I've got little English schoolgirls as children, I've got more family here than in Canada... so after 12 years the ONLY thing that is not English about me is the accent 