I did some testing of my own and set up my environment in kerberos authentication and no proxy. The BBC feed works as expected, but the Financial Times is from what I understand not OK.
I added the link to the RSS-feed in a text file /SiteAssets/ft-rss.txt and got a different error:
An unexpected error occured processing your request. Check the logs for details and correct the problem.
So I did. The logs tells me that the XML provided by Financial Times RSS-feed is not working with SharePoint RSS Viewer.
(GET:http://intranet.template.com:80/SiteAssets/ft-rss.txt)). Execution Time=66,7036
eee47580-33cc-4b53-875c-683fd132784a
06/28/2013 14:06:06.00 w3wp.exe (0x0354)
0x1D74 SharePoint Portal Server Web Parts 8imh High
RssWebPart:
Exception handed to HandleRuntimeException.HandleException System.Xml.XmlException:
Data at the root level is invalid. Line 1, position 1.
at System.Xml.XmlTextReaderImpl.Throw(Exception e)
at ystem.Xml.XmlTextReaderImpl.ParseRootLevelWhitespace()
at System.Xml.XmlTextReaderImpl.ParseDocumentContent()
at Microsoft.SharePoint.WebControls.XmlUrlDataSource.FetchData(String requestUrl)
at Microsoft.SharePoint.WebControls.BaseXmlDataSource.Execute(String request)
at Microsoft.SharePoint.WebControls.BaseXmlDataSource.GetXmlDocument()
at Microsoft.SharePoint.WebControls.SingleDataSource.GetXPathNavigatorInternal()
at Microsoft.SharePoint.WebControls.SingleDataSource.GetXPathNavigator()
at Microsoft.SharePoint.WebPartPages.DataFormWebPart.GetXPathNa...
32c7b0d0-84b1-4011-9617-5f5f869da5b2
I tried to paste, and validate the code in RSS Validator as well ,uploading the source to site assets, and point to the file, but still no luck. The same error emerges in the RSS Viewer and in the logs. So to the best of my knowledge, you can't use Financial Times RSS Feed in out of the box RSS Viewer Web Part. You probably need to build a special web part to make this work.