The alternative is to take US Dollars with you, and then change your money in Colombia, but if you do want to take cash then it's probably easiest to just buy Colombian pesos in the UK and take them. The alternative is to rely on using your bank card to withdraw money in Colombia. There is a decent network of cash machines in Colombia and they take most UK bank cards so there shouldn't be a problem with this, but many people do prefer to take a pre-paid currency card instead.