IF(
    TEXT(Practitioner_Group__c) = "Consultant",
    "C",
    "X"
) +

IF(
    MailingCity = "London",
    "L",
    "X"
) +

CASE(
    Practitioner_Speciality__c,
    "Orthopaedic",      "A",
    "Neurosurgery",     "B",
    "Neurology",        "C",
    "Rheumatology",     "D",
    "ENT",              "E",
    "Sports Medicine",  "F",
    "Pain Medicine",    "G",
    "X"
),

/*Valid Status(S), Consultant(C), London(L) and one of the valid specialities(A-G).*/
"SCLA", "3",
"SCLB", "3",
"SCLC", "2",
"SCLD", "2",
"SCLE", "2",
"SCLF", "2",
"SCLG", "2",

/*Valid Status(S), Consultant(C), London(L) and no speciality(X)*/
"SCLX", "1",

/*Valid Status(S), Consultant(C), not from London(X) and one of the valid speciality(A-G.*/
"SCXA", "2",
"SCXB", "2",
"SCXC", "2",
"SCXD", "1",
"SCXE", "1",
"SCXF", "1",
"SCXG", "1",

NULL