I don't know of a competitive source for replacement Aurora nibs within the UK. Aurora is based in Europe but when I bought my brand new Aurora Optima 366 that had a F nib and asked them if they had a nib replacement service, they said they would need the pen to change the nib and that they would charge me the normal price for buying a new nib.
Aurora Optima nibs just screw out though, so if you find another nib unit, replacing it would be easy enough. Some are yellow gold and some are platinised gold. It would be best to get the same type as your Optima would have had originally.