I know the question relates to UK but we live in Spain and this may be off assistance to others.

Fiancée visas do not exit in The Schengen Area so its either a visitors or spouse visa. I did try, with a former gf to get visitors visa for Spain but even though they was no apparent reason in the eyes of a lawyer) it was refused.

When I met my wife to be, on the advice of a lawyer in Phil we got married and applied for a spouse visa. It was granted as a right (for Spain, which has signed a EC Directive that it is illegal to keep a marred couple separated).