I had a friend visiting from the US who unfortunately developed an infection during her visit, we went to my local walk in centre and apparently emergency treatment is not charged for, she got a prescription which included some additional insulin in case she ran out (as using more with the infection) and i think she should have paid the prescription charge but they didn't ask her when she got it, probably assuming she was exempt due to the insulin in the prescription. So i think they don't get charged for an emergency appointment just if they require hospitalisation, and i think they should only need to pay any prescription charges.