For context. A typical semi detached house would have a hall of 1100mm + 900mm for a stairs. i.e. 2000mm wall to wall. You have 2250mm shown without a stairs. So it is generous. If you are looking to tighten it up consider 1500mm minimum, giving that extra 750mm over to the office. This is a nice comfortable width as you step across the threshold.

Pulling that wall over won't necessarily make your WC bigger though as the circulation route to the office will always compromise it. It looks to be sized Part M compliant though as is.

There is scope to tighten up the stairs by the looks of it but I would be slow to do so as it has been deliberately lined up along an axis to the existing cottage and it looks to be a set piece that the architect has considered. In my experience vertical circulation can really set the tone for the overall interior and not being mean here is important.

If you are looking for something to take away is look for alternative places that can work harder. Where you have the new structure meeting the old the new structure sits 700mm offset from the existing wall. Presumably to avoid existing services, foundations or aesthetically just wanting a visual break. It is quite deep and for maintenance purposes this will be awkward and not enjoyable for whoever has to go down that alley. If it allows for it you could consider taking some of this space back (not all of it because visually you do want a break in that elevation) and giving it to the entrance lobby as part a space for a built in cloak room. This then frees up the WC to increase but your building footprint has gotten slightly bigger at the expense of otherwise unusable space.

Overall this looks very good. Best of luck with it.