If you do it in the UK, its easier. goto your registry office, submit the CNI, takes 21 days, then when you arrive in the Philippines you get it converted at the consul or embassy (prior appointment required, will be ready same day or next day). It is only valid for 3 months from issue in the UK however.
If you do it completely in the Philippines, you have to have been present in the Philippines 21 days PRIOR to making the request, and then wait a further 21 days to receive the CNI after submission.
also bear in mind, that once you have the CNI and submit for the local marriage license, it takes a further 10 days.
