This link shows the flights that are currently operating into Manila. (There's clearly no point in seeking airports for connections if there aren't any flights from there into Manila. The link only shows tomorrow's timetable, not that for early August, but it's a good starting point anyway).
https://www.miaa.gov.ph/miaa/index.php?option=com_content&view=article&id=4348:-flight-schedule-july-06-2021&catid=97:covid-19-updates&Itemid=222
I note, from that timetable, that Philippine Airlines (code PR) and Jetstar Asia (code 3K) are currently flying from Singapore to Manila, so Singapore might theoretically be a possible connecting point. However currently only passengers travelling with a combination of flights using Singapore Airlines, Scoot, Air France, KLM Royal Dutch Airlines, Air Niugini, Garuda Indonesia, Lufthansa, Swiss International Air Lines and Turkish Airlines are permitted to transit in Singapore Airport, so that avenue seems to be closed off. (I only mention it to save you, or anyone else, trying to find connections via Singapore).
I know that a connection via Los Angeles (which is shown on that timetable) won't work as, in answering another question here recently, I've found that the USA won't allow passengers to transit in their country unless they're actually eligible to enter into the USA itself (which basically only applies to US citizens and residents). So we can forget about that route too.
Gulf Air (code GF) though is shown as flying into Manila from Bahrain. That's led me to checking whether Gulf Air can get your family members from Heathrow to Manila, via Bahrain. Taking a sample date of 4 August, I see that there's a flight from Heathrow to Bahrain at 1000, arriving at 1815. The connecting flight to Manila leaves at 2055, arriving at 1130. There appears to be no restrictions barring transit at Bahrain airport. (Indeed, people arriving from the UK are currently exempt from isolating if they're actually entering Bahrain, so there should be no problem transiting between flights which are both operated by the country's own state airline).
Problem solved, perhaps?
https://flights.gulfair.com/