You can ask for what you want, if they want your money be persistent with your request!
Their willingness to do this will probably depend on the build stage.
As for legally I'm not sure, I would imagine it would have something to do with fire escape routes. But if there is another escape route i.e. Patio or French doors I don't see a problem.
I have not come across this and it seems to me to be a badly designed plot, it hard to see from the plan but when you say shared access do you mean with your next door neighbour or open access to anyone walking by?
All that said I find it hard to believe that a fence can't be erected, after all a fence is what 150mm thick at most. Sounds like shite to me!
My question would be why you would accept shared access?
The plan seems to indicate a fairly large house that you are thinking of buying and this privacy/security issue should come as standard.
My advice don't accept anything you are not prepared to live with, it will just fester and annoy you.
There are loads of house builders out there, find a plot that meets all your needs.
Before doing anything, read all the information on this website, it is a absolute must! I'm sure someone will respond with the legality of what you are asking. You should name who you are buying with, this will help people give better advice.
Regards
Ant