actually, a person under the age of 18 can enter a credit agreement but only under certain conditions - it goes back a long time and has to do with waistcoats ( I won't bore you with the details).
A modern day example would be: if a 17 year old has a job that requires him to drive to work and the only option that is available to him is to purchase a car (he lives in the sticks, there is no bus route etc), then a finance company can legally enforce a credit agreement as it can be proved that the credit required is for an essential item and not a luxury item.
It does seem that as you say, all British based companies will not allow a credit card to be given to a minor, but I still maintain that there is no legal reason for this, so in theory at least, a minor can be named as an additional card holder.
In answer to Mrs Pegasus - if a 13 year old came into a shop, offering to pay for goods with a prepaid credit card (as per earlier link), would you be offering yourself up for a case of age discrimination? (whilst I recognise the fact that the owner of an establishment reserves the right to refuse service, if it can be proved that you are only refusing service to 13 year olds paying by credit cards, I think you would be on a sticky wicket - same as if you refused to serve all men for example).