unless I missed something in basic aerodynamics the higher the airspeed the greater the control surface effectiveness hence flap 30 over 40 for strong crosswind approaches
This could be a local policy perculiar to your own operator. If so, it would be interesting to know if Boeing has been advised of its findings in terms of aircraft controllability. Certainly there is no such advice or restriction regarding use of Flap 40 at max crosswind component in the various types of Boeing 737 FCTM or Flight Manuals. In any case the difference in Vref for a given weight between Flap 40 and Flap 30 is only 2-4 knots and I doubt if the extra airspeed would make an significant difference in handling.
Similarly I must say I have not seen any Boeing published document that alters the climb limit weights based on visibility for landing. If there is a British CAA limit on the above then that is your problem I guess.
Re Flap 40 for better cut off angles - that is well known since the first B737-100 was produced.