mysql select everything before a number
As there are only two possible results (one or two characters before the digit) you can do a brute force approach using MySQL's limited regex support:

case when delivery_postcode rlike '^.[0-9]'         -- a digit as 2nd character?
     then substring(delivery_postcode from 1 for 1) -- first char
     else substring(delivery_postcode from 1 for 2) -- first two chars
end