Warning!!
A Terpe long-winded explanation is ahead.

Why?

Because there's no longer any current definitive 'published' answer to satisfy all your questions as precisely as you probably would like and in line with Philippine law.

There are however a number of different regulations that when 'added together' give a reasonable indication of time periods to follow in order to avoid
delays and challenges in the process of getting wed.

A CNI issued in Scotland is valid for 3 months only and that has always been clearly stated.
A CNI issued in England, Wales or Northern Ireland has no specific limit on validity under British Law.
This then is the sticking point. How does it stand up under Philippine law. Some folks will tell you it's valid for 3 months others will tell you 6 months and others will state the non-expiry (as under British Law) still applies.

Bear in mind that the 'local'/Philippine CNI is the one required under Philippine Law in order to secure a Marriage Licence.
This local Philippine version CNI must be used within 3 months from date of issue. Period. (in practice means 3 months validity)

Here's the thing, the British Embassy make a special point of informing that a CNI issued in Scotland must be presented to the British Embassy Manila within 3 months of it being issued.
There's no special mention of a time limit for a British issued CNI.

However, the British Embassy do insist (under the Philippine regulations) that all formal Philippine certificates (birth, marriage, death and no marriage etc) must be obtained from the National Statistics Office (NSO) and must have been issued (on the latest security paper) within 3 months of your appointment.
This then leads to a reasonable and logical (but not specifically stated) understanding that ALL such documents should be presented to the Embassy in Manila within that 3 month window of issue including a British issued CNI

The staff at the British Embassy are the ones who will decide what date of issue to enter and so far it's been difficult to pin them down.
However, concerning Scottish issued CNI the local Philippine CNI will issued based on Consulate date of issue. Means 'daisy-chained' (date wise).

On occasions the Embassy has been known to take the view that the date of issue of the local CNI in Philippines will be the same as the original date signed by the Registrar in the UK. Logically this does not hold up given the current appointment system and waiting time, and also that in following such a practice, it may severely limit the usefulness of the local Philippine CNI.

Under 'normal' conditions there's been no reported problems since most folks actually secure a marriage licence pretty quickly following the issue of CNI from the Embassy.

Based on all the above I'd suggest you ask the British Embassy to confirm in writing exactly what date of issue they WILL use when they give you the all important Phillippine CNI.
Is it the date they issue you with it? or the date that the Registrar in the UK issued it.
As an aside it might be interesting to ask if the procedure they follow is the same for all countries.

Until such time as you receive current definitive legal validity and issue details of UK CNI, I'd personally suggest you use 3 months from date the issue from Registrar in the UK which has previously been informed to members here as date of application (means the 3 months includes the 21 day wait period)

Hopefully those forum members who have been down that CNI path will report on their own latest personal experience on dates etc.

I can tell you the Philippine CNI is strictly 3 months validity
The marriage licence is valid for 120 days anywhere in the Philippines.

Hope that helps your understand and planning.

If you could post any feedback from the Embassy it would be highly appreciated.

For your added information here's the latest (last updated: 07 Aug 2013) published general advice from the British Embassy in Manila - Marriage and CNI Requirements in the Philippines