But basicly, i am hoping to do architecture and environmental design at Sheffield Hallam. This course is validated for part 1, but not for the other parts.Firstly, can i complete the other parts at other unis, and secondly, does my original uni choice/course affect if other unis accept me as a part 2 applicant?
in answer to your question, if you do you Part I at Hallam, you can of course apply to any other university that offers a RIBA validated Part II course. Obviously the downside of this is that you need to apply to these universities, so there is a natural advantage in applications from students who have done Part I courses at those universities (in the sense that you know the system, know the tutors, know they style of work having already done the part I there). I'm not sure how much this matters, but you need to consider it. Some students at universities offering Part I and Part II choose to do their part II at a different university anyway, for a change of scene, style, whatever.
Part III is always done whilst you are working, though you do have elements which are more academic. What I'm trying to say is that you can't do Part III in academia - you need to be working to get it (which is also true for 1/2 of part II).