Word of warning - if applying to come to the UK on either a spouse visa or a fiance visa, the Embassy WILL check with the NSO whether the filipina has a record of a previous marriage. If it's there, the NSO will forward the mnarriage certificate to the Embassy - and the visa will be denied. So any plans to marry in the UK to circumvent the process, if you're coming in on a fiance visa, mean you probably won't get the visa anyway because you are not "free to marry" as required by the rules.
It may be possible to sidestep this by getting a visa in a different category which doesn't require you to be single but, as I think has been posted elsewhere, the 'authorities' don't look too kindly on people who try and abuse the visa process like this - especially the Embassy staff.
If you have a record of a previous marriage at the NSO, there are only two ways round it. (1) - get it annulled or otherwise 'removed' from the NSO (sadly, anullment is a lengthy and costly process and you are at the mercy of unscrupulous family lawyers who know they have your balls in a tight grip and will keep squeezing - even the most reputable ones - this is the Philippines after all) (2) if you have been living together for 2 years or more, you can try applying for a settlement visa as a cohabiting couple who are unable to marry. This type of visa was set up for gay couples originally but Human Rights and Equalities legislation wouldn't let heterosexuals be excluded from this privilege - (god bless Europe, occassionally), and so a male and female couple can use this route if there is a good reason why they cannot get married. A country with no divorce and, legally speaking, very strict and limited scope for anullment (not that the Philippine courts seem to care much about the law when they're granting anullments, it's often more to do with the quality of lawyer you get (ie, most expensive, who is friends with the judge, and better friends than the other lawyer), means you arguably have a case for being "unable to marry" - however, I ahave no knowledge of this argument ever being tested so I don't know what the Embassy would think of that.
If you have kids (between you, not from the previous marriage) then the Embassy is duty bound to consider that and it may well help an application under this route.
Only suggestions - I have no idea whether these arguments would work, but like everything else, the better presented, the more chances of success I would imagine.
The only thing I'm pretty sure about is that the Embassy will most likely find out from the NSO if a previous marriage exists. Whilst local registrars won't bother checking (they're happy to take your cash and 'marry' you, come what may, the Embassy probably will and you'll then be stuck in the unenviable position of being 'married' (with an NSO certificate to prove it, which can be stated as valid in the Philippines), but in a marriage which the Embassy will not recognise as valid.
