Once you've double checked that you have entered your details exactly as they appear on your card; if you are still experiencing problems, please contact your bank directly and they should be able to identify the issue.
2000 Do Not Honor - This means that your bank is unwilling to accept the transaction. Usually it's just your bank protecting you. For example, you might have been using your card more frequently than normal or if you have never ordered from a UK store before. Please contact your bank for more detail.