Quite funny that Uk complains today about enlargementn of EU when they defended it fiercely some years ago...
I think that if UK leaves EU the first think they will do is try to renegotiate the acces to the european single market. Acutally I would not be suprise if two years later UK will have almost the same obligations that if the country was eu member. What would probably happen is that UK will have to adapt his laws/policy to have the right to sign bilateral agreements with EU but without having an influential say on EU laws/policy because UK would not be a EU member.
However I hope that EU will not give any privilège to UK and show that leaving EU has price. I believe that in short term it will be a true challenge for EU but if EU survives well at leat that will maybe be the end of members with speacial rights. I hope that in the future all members will have the same rights and duties inside EU. Maybe it is a good thing in long term for EU if UK leaves.