Your wife's children can come here in UK without their mother flying there. As long as you get the proper documents. Just go to the Philippine Embassy and let them stamp the firm which is Affidavit of Support and Consent to Travel and Special Power of Attorney, this means that the mother has the sole responsibility to her children. After that get a DSWD Travel Clearance for Children together with that Affidavit from the Philippine Embassy. Then you can book the plane tickets of your wife's children. You can book them as unaccompanied minor but you have to pay their ticket as an adult fare because the flight attendants will look after them throughout the long haul flight. My friend did all this because she cant come home due to heavily pregnant 3 years ago.
I am only sharing my friend's experience towards minor travelling alone. But it is still up to you what you will do in the end. Good luck to all.