Flat doors do not have to open outwards.
It is worth noting that flat entrance doors must be British Standard Fire Doors fitted with self-closers, fire strips and close fully (check lease for responsibility of flat front door as may be Management Company or Leaseholders).
The door of any communal doorway or exit should, if reasonably practical, be hung to open in the direction of escape and, as AnthonyB says, should always do so if the number of persons that might be expected to use the door at the time of a fire is more than 60 (the door should be changed to open outwards even if the number is less than 60 where there is a very high fire risk)