Hi
Here’s a possible solution for you (it’s maybe “overkill”, but quite flexible - if you ever need any other postcodes), using some database programming.
I have used M1-M18, then 1 digit (0-9), then 2 letters.
I’m not quite sure which “double” letter combinations are valid (at the end of the postcode), so my query generates all 676 possibilities (AA-ZZ).
I got 121680 combinations by doing this. You can get an evenly distributed sample by using the last query.
I know that it would be a tall order to understand and run the code if you are new to databases - but maybe this type of coding is not a problem for you 
Cheers
Stefan
-- ----------------------------------------------------------------------------
-- create and populate 3 small tables
-- ----------------------------------------------------------------------------
-- pc1: hold the first (couple of) letter(s)
create table pc1 (
prefix varchar2(5) unique
);
-- pc2: hold all possible digits
create table pc2 (
digit varchar2(1) unique
);
-- pc3: hold all possible letters
create table pc3 (
postfix varchar2(1) unique
);
-- populate the tables
begin
for pre in 1 .. 18
loop
insert into pc1 values ( 'M' || pre );
end loop;
for digit in 0 .. 9
loop
insert into pc2 values (digit);
end loop;
for i in 1 .. 26
loop
insert into pc3 values ( chr(i + 64) );
end loop;
end;
-- ----------------------------------------------------------------------------
-- SELECT all possible postcodes
-- ----------------------------------------------------------------------------
select prefix || ' ' || digit || letters postcode
from pc1,
pc2,
( select L1.postfix || L2.postfix letters
from pc3 L1, pc3 L2 )
-- postcode count
select count(*)
from pc1,
pc2,
( select L1.postfix || L2.postfix letters
from pc3 L1, pc3 L2 )
--> 121680
-- ----------------------------------------------------------------------------
-- use a sequence for numbering the postcodes, and store them in a table
-- ----------------------------------------------------------------------------
create sequence pc_seq
start with 1
increment by 1
create table postcodes
as
select pc_seq.NEXTVAL id, prefix || ' ' || digit || letters postcode
from pc1,
pc2,
( select L1.postfix || L2.postfix letters
from pc3 L1, pc3 L2 )
-- ----------------------------------------------------------------------------
-- select how ever many postcodes you need
-- (change the number in the mod() function call)
-- ----------------------------------------------------------------------------
select * from postcodes
where mod(id, 1000) = 0
