The "bating" in "combating" is pronounced the exact same way as "batting". It doesn't make sense to me.
Generally, if the base ends in a single consonant represented by a single consonant letter, the letter is doubled before suffixes beginning with a vowel, subject though to the base being stressed on its final syllable. Monosyllabic bases do of course have the stress on the final syllable and hence always satisfy that requirement: "fat"/"fatt.er"/"fatt.est" ; "bat"/ "batt.ed"/"batt.ing"and so on.
But the verb "combat" can be stressed on either syllable so the suffixed forms are spelled with or without doubling: "combated"/"combatted"; "combating"/"combatting".
Normally consonant doubling (CD) occurs after short stressed vowels, and after long stressed vowels + r (-ar/er/or/ur) as in to prefer, preferred, to star/starring, but to stare/staring.
CD with unstressed syllables is a kind of grey area. It occurs regularly after -el as in to travel, BrE travelling, AmE traveling, and after -al as in to signal, BrE signalling, AmE signaling.
Sometimes it occurs after p as in to kidnap/kidnapping, to worship/worshipping, but not in to develop/developing.
Unstressed syllables ending with -t normally have no CD. But to combat is a special case. Pons has to combat and combatting/combating. Perhaps the CD is occasional in analogy to to bat/batting. Oald, Collins, McMillan don't mention CD.
I believe that the English UK spelling should be combatting in all cases, according to spelling rules, as someone pointed out above. This word is no "exception to the rule": the confusion stems simply from erroneous MS Word spellcheck, which for this word has US settings even when you are writing in English (United Kingdom)...