Any thoughts welcome.
If it's possible put it above the shower and duct it through the ceiling. More from an aesthetics perspective. And fan should be able to shift the air in that size room pretty quickly so placement for efficiency is less of a concern.
If you use that window, I would be wary of B or C because the fan will pull air in through the window rather than dry air in through the door which is what you are aiming for. I also wouldn't want it near my boiler tbh. No idea if it would fall foul of regs if it is in B or C but from an access perspective it could make things difficult and you've got more chance of fucking something important up when wiring it.
You know about bathroom splash zoning presumably? You might have to use a 12v fan with a transformer or a zone 2 certified 240v.