This is covered in Building Control Regulations Part J (Heat producing devices) which you can download from here.
http://www.planningpo...uments/partj/approved
There's lots to read; you were very unspecific about what you are trying to do (i.e. does the question relate to the physical position of the boiler or the flue outlet, or flue length or angles or all of these things) so if you specify what you are doing someone will be able to help you further.
We are considering moving the boiler to the downstairs loo. It doesn't have an exterior wall but the flue could run through the wall via the utility room to an exterior wall.
I wasn't sure if this would be an acceptable location.
Contrary to popular belief they dont have to be on an outside wall although some heating engineers say otherwise but that seems to be for there benefit as they are easier to install, not all engineers are happy about messing around on the roof putting vents in...