http://www.sephardikashrut.org/restaurant.htm
Here's the SKA's list of restaurants.
Sadly Bevis Marks doesn't seem like it is Glatt, but several others on the list are.
Alternatively, if you want to pick up some nice take out food close to the centre of town, you could try Adafina delicatessan:
http://www.adafina.co.uk/
It's in St John's Wood, on the same road as the Beatles' recording studio and the famous zebra crossing pictured in the "Abbey Road" album.