It's a real pain, I've had to leave town without parking on a couple of occasions, and not only when pregnant! There are 10 family parking bays in our whole town, and because there is no legal recourse if people who aren't eligible to park in them do, then everyone with a van / huge car / dog / lazy attitude parks there. One of the reasons I wanted our current car is that the passenger seat folds flat, and the back doors slide open, so I figured I could always climb into the back and get out that way!
Last pregancy, I took to sticking a notice in the drivers side window on an A4 sheet, in dark, noticeable marker, saying "Pregnant driver, please leave space to open door, thanks". This seemed to have some success when parking next to an empty space - people did seem to leave room parking next to me. Not sure how it would go in London, and not much good if the space is too small when you park in it, and you can't get out to begin with.
I always try to park in an end space if available - I'm sure you do this anyway. There are also some freak spaces in our local car park, where there was more space than for one car, but not enough to divide into two spaces - I've learnt where these are now (sad, but we have a VW Sharan, so necessary!).
If I see muppets with no children have parked in the family spaces (i.e. I witnessed them parking there), I have been known to leave a note on offending cars, saying "These parking spaces are intended for parents with young children - please respect this", or politely tell people the spaces are for families. Depends how likely the person involved looks to knife me / how stroppy I'm feeling. My auntie did this once and was told to "fuck off" right in front of her granddaughter. Nice. I've also spoken to the car park attendant about people abusing the spaces, and politely asked him to have a word with people he sees doing this.
Not much else I've been able to think of. Sorry. Would like to hear any other ideas though.