Fuse specs are different between the US and UK. The US says a 10 amp fuse will take 10 amps forever. The UK says a 10 amp fuse will blow immediately at 10 amps. The difference mans that a UK 10 amp fuse is equivalent to a US 5 amp fuse. I've attached a document that explains this better than I can.
I think if you add any circuits, I would fuse them independently. As far replacing wires, there are very few places on a Europa that need to be bigger than 14 gauge so I would use that size wire for those places.