There are several pre pay calling cards available for making international calls. Basically, you dial an access number (often free phone) and then enter your card number, PIN, and desired phone number. You are then charged from the credit you put on the card.

ekit are a common option - it is this that comes free with the VIP Backpackers card. Charges for calling the UK vary from $0.33 to $0.66 per min, with a $1 connection charge. I personally think that its easier just to pay $0.49 per min from a Vodafone mobile - the range of access numbers can be confusing and if you cannot get through on the cheaper numbers, then it costs more than the mobile option. Other things to be aware of is that it may cost more from a mobile, and otherwise you need a landline to make it work. Finally pay phones may require a deposit for dialing a free phone number.