West Hampstead Thameslink or Kentish Town <> London Thameslink (St. Pancras, Farringdon, City Thameslink, Blackfriars, London Bridge or Elephant & Castle)
Valid between West Hampstead / Kentish Town and King's Cross St. Pancras, Farringdon, Blackfriars, London Bridge or Elephant & Castle on LU (but NOT where tickets are routed 'FCC ONLY', 'LDN NOT UND' or 'NOT UNDERGROUND')
My quick perusal of BRFares.com leads me to believe that a Zone U1* fare will actually be issued. However, looking at the interavailable routes it appears that this section is interavailable so it will be fine.
I understood what maniacmartin was saying to be that a ticket issued to ZONE U1* LONDN would be valid on National Rail from St Pancras to Farringdon because of the interavailability with National Rail-issued U-zone tickets on that route. But I think he may have used the wrong quote.