If this first flight is not be followed by an every day flight test, the weather has nothing to do with this, they have now to carry out some amount of checks to insure the good healt and safety of the engine and the modified aircraft. And this can be done in Cambridge what ever the weather condition are.
Moreover, go over the ocean for a first flight would have been too risky and would have needed to flight much more than 1 hour.
Up to now, the weather has never been the reason for delays and slow progress on flight test program.