A&E departments try and follow patient wishes, but are guided first and foremost by the Hypocratic Oath. They will resuscitate you unless they are absolutely sure of your wish to be DNR.
Your home made card may not be sufficient to convince them to withhold treatment. As the law stands at the moment a donor card (which, C-C, is not what Pharlap was referring to) would only indicate your wishes, but your next of kin still have to agree.
You must tell your next of kin and close friends what you want them to agree to and make sure they can find at least a certified copy of your Living Will quickly in the event of them being called to the hospital in an emergency. They will almost certainly have to show it to a doctor to persuade him to withhold treatment.
You should make sure your GP is aware of your wishes too. If you require long term care (which for those of you that don�t have one is really what a Living Will seeks to control) he/she will be involved in that process too.
Hope this helps.
AG