I don't entirely understand the functionality you want from a dressing room -- is this basically a walk-through closet with seating, or something more/other?

I want it to be a walk through closet with plenty of storage - the fact that there is a window there means i cant put anything high against that wall so think the best thing to put there is a small dressing table, or will this limit the wardrobe space too much?

Well, you can't do much about a window! Dressing table is good, as well as hooks, and/or shelves on the wll to the right of it. About all you can do for regular closet storage will be all along that (newly) straight wall to the left of the window, because everything else is constrained by the needs of the pass-through. You might be able to run a shelf along above the two doorways, for a little extra, but it could end up feeling a lot like a tunnel for the entrance to a master bedroom. I'd suggest making at least half of ht ehanging space be double-rod (half-height) for more capacity, and make sure that part of the dressing table has some drawers. That's about all there is! :)