Do you have a preference for "fungi"? I say [fVndZaI], not [fVNgaI]
("fun-jye", not "fung-guy"), so it's a "soft 'g'" in fungicide too. It
seems reasonable to keep to the same convention in both.
For the record, I consider the "soft 'g'" correct before front vowels in
learned words (most of which have at least passed through Latin),
because that version follows the sound changes that were made to Latin
words before they were borrowed into English; but many people,
especially young ones, use the "hard 'g'" in unfamiliar words and will
think you're odd if you don't too. You can't please everybody, so you
might as well please yourself.
I should add that I'm Canadian, so not British but tolerably cognizant
of British usage.