It could only be used as a single bedroom, but you would need building regs approval and freeholder consent as above, which you may not get for a flat.
Yes its the size. Although the space standards in Part X of the Housing Act 1985 are advisory in some circumstances, if the property were ever inspected by the Council, then they would almost certainly use their powers under HHSRS to prevent a room so far below the 10.22m threshold for a double being used for 2 people. If the flat is in a Selective Licensing area, it would fail the requirements for a licence.