DHL have confirmed that this issue is now resolved. Future orders will book with no issue and any failed orders will just need to be retried. Please reach out to support if you have any issues from there.
We have had reports of users receiving errors on their DHL Parcel UK orders; that being they're failing with the error "Bad Request". This is something we've reached out to DHL Parcel UK about and are awaiting an update from them on the current situation.