It appears that this is a British/American distinction. The Corpus of Contemporary American English (COCA) reports 92 incidences of “fill out a/the/this form” and just 2 of “fill in a/the/this form”, clearly establishing “fill out” as the standard idiom in American English. I haven’t worked out how to search the British National Corpus yet, but I wouldn’t be surprised if the results were reversed there.
OK, I got the BNC to respond to queries—although it sure takes its sweet time—and I got 19+7=26 results for “fill in a/the form”, and 5+1=6 for “fill out a/the form”. So it does appear that British English favors fill in over fill out, although not to the degree to which American English favors fill out over fill in.