A single consonant letter representing a consonant sound following a single vowel letter is doubled.
'Slow' is not an exception, in that sounds represented by the letter w are not phonetically consonants. Phoneticians class /w/ as a glide or semi-vowel. It shares some of the characteristics of a consonant; for example, nouns beginning with /w/ follow a, not an. However, there is no consonantal obstruction of the airflow when /w/ is produced. The sound after /sl/ in 'slow' is a diphthong, /əʊ/ in BrE; there is not a trace of a consonant sound..