Japan retaliates, America doesn't get involved (unless enough PR is used to spin it); Japan shifts its strategy to a AHC Eastern Strategy (instead of OTL North or South), they run up Malaya, Burma, and cut Chiang Kai Shek off. Overall a nightmarish situation for Britain, because there is no guarantee the US will enter the war if Britain starts it.
That being said the Japanese could overstep, the Philippines do become a major thorn to the Japanese. Additionally, they may attack the Dutch East Indies anyway and this may incite the US.
If they don't overstep; they gain more time to prepare for their potential fight against the US.
Final Verdict: It would be stupid of the British to consider a pre - emptive strike against the Japanese considering the events in Europe.