Fiancé visa vs spouse visa

Firstable thanks for any advice.

I'm 25, Colombian Doctor. My boyfriend is a British citizen and we’re in a hard situation at the moment.
I came to the U.K under a turist visa aiming to extend my stay for a couple of months due to sitting the PLAB test. (On the apendix V of the visitor rules there’s a paragraph were sitting the PLAB test it’s enough reason to extend the visa). I called the home office to confirm this information and they said it’s not possible to extend it more than six months. My current visa goes from the 18th of Aug to the 18th of February and I’ve booked the exam for the 15th of March which was the next available date.

Im the first place we didn’t want to apply for any other visa before as we wanted a planned marriage (not extravagant but at least with a nice ceremony and party) and we’re relying on the fact that I’d be able to extend my turist visa which would give us more time together.
At the moment I’m really shock by the news that I received over the phone call. But we still want to be together, being away from your other half is a torture.
I came across this forum and I wanted to get some advice: first it looks like my only option is going back to Colombia and applying from there for a fiancé or spouse visa, I was thinking about getting married before leaving the U.K on this visit but I’ve read that’s not possible under a turist visa and it could affect my future applications for an spouse one. Is it better to apply first for a fiancé and then apply for a spouse ? Does it have any difference ? Can I go back to my country before my current visitor visa expires and apply immediately for another type of visa?

Thanks for your help.