Welcome to the forum, Andy. In certain respects, your situation "mirrors" my own.
After a year of almost daily correspondence with a Filipina High School teacher, I visited her home province of Davao for 3 weeks last September - during which time, we decided we'd like to spend the rest of our lives together, and she applied for early retirement. I then came back to Scotland in order to acquire a Certificate of No Impediment from my local Registrar in Perth, before returning to the Phils in late November. A few weeks later, we were married there in a civil ceremony, and I twice extended my stay to await the processing of my wife's visa. She finally arrived in the UK on March 8 - exactly three months to the day from our wedding date.
Since it would appear that yours' too, is a genuine relationship, then I would strongly recommend you opt for the spousal route. Visit her a fourth time ["armed" with your CNI (ensuring she obtains HER Cenomar in the interim!)] get "hitched" in her homeland, and start the ball rolling ...
... the reason I'm suggesting dispensing with any formal engagement is because, in my opinion, it's an infinitely more expensive choice. The cost of a *fiancee visa is precisely the same as a Spousal one ... both £550+ ... and then there is the *added expense of handing over this amount [at least] AGAIN to apply for FLR (Further Leave to Remain) if you marry over HERE, as the law, as it stands, only allows her to stay in the UK for a maximum period of six months (within which period, the wedding MUST take place).
That's my advice for what it's worth, mate. Others - for reasons best known to themselves - might disagree. But, if I were you, I'd follow my own instincts[as I DID, in the end ... once I'd been made well and truly aware that the tourist/visit method (which I'd wanted to begin with) was likely to prove a non-starter]. And I've never regretted it. So, if the pair of you are really serious about one another, then GO, go, go ...
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