Hello Jessie, welcome here to the forum
Concerning your question here's what UKBA stateMy advice is to comply with those requirements as far as you can in order to minimise any risks of refusalAll supporting documents must be originals, not copies.
If you have any problems to meet that then consider using properly certified copies or true copies of some documents ordered via your local registry office.
Don't send your passport, just use a certified copy
Try to spare some time to review this guide on How To Apply for A Fiance(e) Visa
Please don't think I'm trying to put you off, but with the fiance(e) visa do give extra focus to providing evidence of your genuine relationship and especially of your intentions to get married here in UK