In South America, VISA is the best... Never had a problem with HSBC and a Barclays combo there..
To be safe, spread your travel cash between two bank accounts and take two debit cards... Visa is best and probably what you will get for a UK bank account anyway..
Get yourself at least two credit cards for anything you need to buy online and one for an emergency... Make one a MasterCard just in case. Keep one credit card stashed under your seat (this saved my ass BIG TIME when I lost my wallet in the middle of the Ruta 40, Patagonia...)
Venezuela is a nightmare for ATM's so make sure you have a fat wedge of USD to change at the border. It's always good to keep $300USD stashed on the bike with the credit card too. Wrap it up in a plastic bag and gaffer tape and keep it hidden and forgotten about..
Most importantly... DO TELL YOUR BANKS YOU'RE TRAVELLING !!!!
Oh, and get yourself a muggers wallet (an old cheap wallet with expired cards and a few dollars in to hand over to muggers/bad cops for bribes etc)...
To keep fees down, I would just visit ATM's as little as possible. Get the maximum you can at every withdrawal... I looked at loads of pre paid travel cards etc. They always seemed to give crappy rates and people I knew who had them could never get them to work outside of the capital cities.
2001 NX650 Dominator.
EX - BMW Dealer Technician