You could get PHP here but only enough to get you from the airport to where ever you are staying.
Take GBP with you if you feel comfortable carrying large amounts of cash and can find somewhere safe to keep them on your arrival. The money changers give excellent rates with little if any commission.
If you are going to use ATM's when there be aware that maximum withdrawal in many is only 4000 PHP. You can use the machines at the banks in the financial district of Makati where you can withdraw up to your daily limit.
Most banks will charge £2-£3 for each withdrawal so if you are staying in Manila a trip to Makati is worthwhile.
The Nationwide is the only bank, to my knowledge, that does not charge for withdrawals when abroad but if you do not bank with them then I do not think you have time to open an account before you depart.