British vs American spelling

Got my final draft back and am making the last changes I need to do before taking this damn thing to the bindery. I have quite a few pictures to add, annoyingly enough, plus the raw data and stats tables that go in the appendices. However, one thing my supervisor marked has me unsure.

To to Brits, Irish, Scottish, and anyone else on my flist who uses UK spelling, how do you spell what in America is "defense"? My supervisor wants "defence", which I hadn't really seen before. Word, although I've set it to UK spelling, didn't mark it wrong when I spelled it with an S, and though I've found it with a C in a few online dictionaries, I'm curious about how common it is.

[Poll #847799]

If you spell it with a C, does that make it "defencive"? That just looks weird to me.

I'm still deciding which of his advice to follow and which to ignore. He marked out my Oxford commas, the bastard. I like Oxford commas. They help get rid of sentence confusion.

Another query...

Okay, I've read this sentence so many times I'm no longer certain what to do with it. My supervisor was confused by it and at first suggested I'd left out a word, but once I explained it he said I might want to reword.

I'm talking about chemoreception and how a previous study did some bad science, skipping ahead to test the ability of cuttlefish to distinguish b/w two stimuli without first testing that they even have a sense of smell (distance chemoreception). I wrote:

"In this case, the researchers moved ahead to test the fine-tuning of an ability S. officinalis were not proven to possess."

This sentence makes perfect sense to me. However, it obviously confused my supervisor. What say ye, flist? I'm trying to think of a suitable rewording without repeating myself and making it sound stupid.