Working near Regent's Park - Where to live?

On the Bakerloo line, I'd go with Maida Vale or Queens Park.  It's a lovely place.  On the Northern line, I'd suggest Belsize Park or Hampstead.  On the Metropolitan Line I'd look at South Hampstead (near Finchley Rd station).  I've lived in NW London for all five years of being here and LOVE IT!

Most of us in London can't afford that.

I guess it depends on your line of work and your lifestyle.  If you have a well paid job and you are single or with a willing partner you could easily move near to work, especially if renting.
What do you do if you're made redundant or you want that new job that is on the other side of town?  I guess you'd have to move with the job - lease allowing.  If you own your house perhaps not so easy.
Or if you do consulting work and you're at different locations every month or few months it'd be very difficult - but then again, I guess you wouldn't choose that line of work if you had to be within walking distance.

Not having to pay for a Tube pass saves a lot of cash too.  And actually living in Tower Hamlets is no more expensive than many other parts of London.