Tea and Kosher
This is one kosher restaurant in the Marylebone area.
79 Baker Street, London, W1H 2JR
Telephone: 0871 3327840
If you go to www.london-eating.com, there is a page of 'Jewish' restaurants. Most are in the northern part of London, but there are also others. I think one of my kosher friends has been to the above place, but it was awhile ago.
Kosher options are limited in Central London.
As well as Reubens mentioned above, there is 613 on Wigmore St and the restaurant attached to Bevis Marks Synagogue.
As far as a kosher 'tea' is concerned, as far as I know there are no 'milchik' places in central London, but they can of course be found in Golders Green and Hendon.