Hi Jenny and welcome! A couple of questions before I could really provide what I think is a semi-suitable answer:-
1. Is your work permit due to expire soon?
2. How soon do you and your fiance plan to marry?
Reasons why I ask is because a fiance visa only gives you leave to remain in the country for 6 months. You and your boyfriend must marry by the time the 6 months is up. Also with a fiance visa, you are not allowed to work. So, I would say you would need to look at how much time you have left on your work permit as opposed to how long you intend to be engaged.
On the surface, I would say you really probably don't need a fiance visa. You're already here. You're already working on a work permit. It seems to me you don't really need a fiance visa because it will not be of any benefit to you to have it. In fact, quite the opposite, it may end up being a detriment. I'm also not sure that the Home Office would even grant a fiance visa to someone who's already here on a work permit AND, someone correct me if I'm wrong here, but I don't think the Home Offices in the UK actually grant fiance visas. It's been my understanding that fiance visas were only granted to those outside of the UK who planned to come here, get married and settle.
Hopefully this does not confuse you, but sheds some light. I would say you don't need it and that once you are married, then just apply for a spousal visa instead.
Best of luck in whatever route you decide to go!
