Hello Inday,
So sorry to read about your fiance, but God is always good. And I know that He is just waiting for an opportunity to do wonders for us if we let Him. I know this by heart, cos I've seen Him do wonders for me and I know He wants to do the same for your and your fiance. May the Lord provide you strength and comfort during this hard times. Just keep your heart still God is faithful and He is our greatest healer.

About your desire to get married while on Family Visitor Visa, erm this link won't be helpful at all https://www.gov.uk/family-visit-visa/overview cos it just provide things of what you can't do sadly. Although, I think you should still seek help and hopefully somebody here in the forum can help.

Uhm, can you get married on Schengen visa? Maybe you can look that up. First if you can apply while you're there.. and if you can get married outside UK. But I still don't know what you can do after you're back. Anyways, it's just a suggestion. Might be worth checking.. God Bless