The sword! The sword!
Using a letter opener would look rubbish unless you were having a 7" Victoria sponge from Tesco
However, be warned, repro swords are blunt (and heavy). I've got a full sized sword and a druid's sickle and both wouldn't be capable of cutting jam. You may end up ruining the fancy icing on an expensive wedding cake. You could get one sharpened just enough though? Or buy a real blade - my dad had a Napoleonic one complete with nicks where it probably cut some unfortunate Frenchmen down in the field.