Rustle:
I think you'll find that since the tragic accident at Blackbushe (private Kingair crashed on takeoff in poor vis) the UK CAA would not take too kindly to people departing in O/O. Also licenced airfield operators are now required to state in their Airfield Manual the weather conditions below which departure is not permitted.
IO540:
The way I read it is if you have no ops manual saying you can go in less than 400m, then 400m is the minimum. If you're private then you have no ops manual.