Hi Cebu
I understand your situation somewhat better now, it appears you live in Philippines, but are not married as yet, also your baby does not exist as yet, I thought that is what you have said.
However if your prospective wife is pregnant by the time you apply to enter the United Kingdom, then the question you posed earlier is academic, and would not apply, if your child is born in the UK, then normal procedures for registering a local birth apply, and as you as the Father are a British citizen by descent, then it should be taken for granted that your child would be entitled to a British passport.
However, some people on this forum get emotionally involved when it comes to rights of settlement, and that clouds the issues.
People who have well meaning opinions often tend to cloud the issues, the issue of settlement is far different from the issue of rights to the protection of the British Government which in fact is the only reason to obtain a British passport which is in effect a travel document.
British travel documents by nature of the very complex aspects of the former empire and those ones who may posesses a British travel document, but in reality do not have the right to settle in the UK, can sometimes be an object of frustration to those ones who assume sometimes wrongly, that being entitled to one document, then gives them the right to enter the UK and live.
For example, if you have been in a relationship for 4 years or more, then in your case, the situation is diferent, than for someone who has met recently over internet and has known each other for a limited time, say 3 months or so.
But, the matter of settling in the UK, is a matter of firstly, financial matters and seconly, accomodation matters.
This is because the UK government does not want to issue settlement visa's to people who cannot support themselves in the UK without the need to apply for state benefits to help in their living costs.
Therefore, should you wish to marry in the Philippines you may do so, and if your wife to be is pregnant, when you arrive in the UK, provided she arrives by virtue of a settlement visa in her name, then access to the National Health service is ok, and she should be able to have her baby on the NHS without paying any costs whatsoever.
Some foreigners however do arrive in the UK, and are sick at the point of entry, there have been numerous documented cases of such ones, being asked if they have insurance or other means of paying for their NHS care, some have been presented with bills of £1000.00 or more for care, however in the mainstream, its my understanding these bills are rarely paid, because the NHS is not structured for private fee paying patients.
In your case I am sure, you will be able to do this without any concerns, whatsoever, firstly though, on marriage, should you wish to return to UK, ensure that you have not been out of the country for too long, as after 12 months of being out of UK, your right to any entitlements becomes more difficult, such as Job seekers allowance and Income Support, in any event, your settlement visa would be refused if you sought to rely on those benefits.
That means, you must have a steady job in the UK, or private and independant financial means to support yourself and your wife and any child in the UK for up to 2 years, if you have plenty of savings, say in excess of £5,000 to £10,000 and can prove this, you may find that having a regular job or income may not be as important, however, having a regular job, income and proof of such will make your application for a visa for your wife and any child much easier.
Secondly, there is the matter of accomodation, the entry clearance officer who reviews your application would want to see that you can accomodate your family in the UK without attempting to rely on local housing stocks.
Again, these are questions you must ponder, and should you satisfy the requirements, then your passage back to UK should not present you with any problems, I hope it goes ok for you, and wish you well in the future.
But just a word of caution, there are thousands of people living around the world, whose children have british passports, because of the many categories of passports, it does not mean they have the right to live and work in UK.
For example, there are British protected persons, British Overseas Citizens, British commonwealth citizens, they have the protection of the British Government, but do not neccessarily live in UK.
Best wishes...
Pete