Those coming to the UK

[LEAHnew]

and if coming on a fiance visit - are you entitled to free NHS treatment?

I'm not sure what kind of NHS treatment you wanted but as a fiancee holder IMO you can register easily. In my experience just after 3 days I arrived here in UK (as fiancee visa) and been registered quickly and got my NHS card after a week, and had my first visit to GP already btw it's a family planning concern and got free pills as well
As fiancee you can use this NHS card for your identification for marrying purpose and applying FLR