It is only necessary to stay in Phils for 21 days if your fiance applies for a CNI (Certificate of No Impediment) in the Philippines.
Your fiance can get a CNI from his local registry office in the UK, it will cost about £30 and after applying, he will have to wait 23 days for the CNI to be issued. He can then exchange the British CNI at the British Embassy in Manila or the British Consulate in Cebu, for a Filipino CNI as soon as he arrives in the Philippines. Then you only have apply for a marriage licence which varies from province to province, but usually takes 10 days. Once you have the marriage licence, you can marry whenever you want.
It is advisable for your fiance to bring his birth certificate with him as it may be required when you apply for the marriage licence and if he is divorced, he will also need to bring his divorce decree absolute document.
Iain.