According to the Companies House website, their UK address is listed as Room 3508 10 South Wing Office Terminal 3, Hounslow, Heathrow, TW6 1JH. All companies registered in the UK must have a registered address and they are supposed to list their principal officer, which Philippine Airlines does not. Their BR number is 000740. Interestingly the last change of address was "Details changed for a UK establishment - BR000740 Address Change Aviareps house gatwick road, crawley, west sussex, RH10 9RB, 25 April 2017" four years ago.
So sending the registered letter to this address is the first stage, presumably to tell them that you are about to serve them a Statuary Demand and see if anything happens.
Best of luck